The Syrian government pounded rebellious areas east of the capital, Damascus, early Wednesday, and antigovernment activists said some rockets included chemical weapons that killed scores of people, and possibly hundreds.

Photographs and videos showed rooms full of lifeless bodies laid out in rows, some wrapped in white cloths, others lines up in mass graves. Some showed victims staring and motionless, others twitching uncontrollably.

The Syrian government vociferously denied mounting any chemical attack, and its ally, Russia, blamed Syrian rebels for launching a rocket with an unknown chemical agent that had caused civilian casualties, calling it a preplanned effort to accuse the government of President Bashar al-Assad of using chemical weapons. A team of weapons investigators sent by the United Nations arrived in the country on Sunday to begin looking into several other reports of chemical weapons.

The total death toll remained unclear, although though the images, along with testimonies provided by antigovernment activists and medical personnel, indicated at least scores of victims, including men, women and children. Some opposition estimates went as high as 1,000. The Syrian Observatory for Human Rights, which follows the conflict from Britain through a network of contacts inside Syria, said the attacks took place in the suburbs of Zamalka, Ein Terma and Erbeen, all of which are east of Damascus and have a strong rebel presence.

The White House said in a statement that it was “deeply concerned” by the reports and was still trying to verify what had happened but “strongly condemns any and all use of chemical weapons” and called for a United Nations investigation and Security Council consultations.

“Those responsible for the use of chemical weapons must be held accountable,” said the statement, issued by Joshua Earnest, the principal deputy White House press secretary. Other countries, including Britain and France, offered similar expressions of concern.

Unlike the videos often uploaded by all sides in Syria’s civil war, the images on Wednesday showed very little blood. Few of the victims appeared to have conventional injuries; most appeared to have suffocated.

In one video, medical workers move through a room full of apparently lifeless young men in their underclothing. In another, about 80 bodies, including those of about a dozen children and many women, lie on a tile floor.

Gwyn Winfield, editor of CBRNe World, a professional journal that covers nonconventional weapons, said the images suggested to him that either a large amount of a crowd control agent like tear gas was used a in a confined space or a weakened form of a more powerful chemical agent was used.

It was not clear whether the team sent to Syria by the United Nations would be able to investigate the new reported attacks.

Secretary General Ban Ki-moon said that the team was “in discussions with the Syrian government on all issues pertaining to the alleged use of chemical weapons, including this most recent reported incident.”

Eduardo del Buey, a spokesman for Mr. Ban, would not go beyond the statement in response to questions over the team’s ability to investigate the reported attack.

The team was to begin working on Monday. Numerous allegations of chemical weapons use have surfaced during the civil war between the forces of Mr. Assad and rebels seeking his ouster, but evidence has been disputed.

The White House said Syria should provide access to the United Nations. “If the Syrian government has nothing to hide and is truly committed to an impartial and credible investigation of chemical weapons use in Syria, it will facilitate the U.N. team’s immediate and unfettered access to this site,” the White House said.

In past instances of reported chemical weapons attacks in Syria, whether by government or by rebel forces, it took week for the American intelligence community to gather evidence and conduct tests sufficient to offer a credible assessment.

The Russian statement, issued by Aleksandr K. Lukashevich, a spokesman for the Foreign Ministry, said, “A homemade rocket with a still unknown poisonous chemical substance, the same as one terrorists used in Khan al-Assal on March 19, struck this area early on Aug. 21 from a position held by insurgents.”

Noting that local news organizations immediately blamed the Syrian government for a chemical attack, the statement said, “All of this can only suggest that once again we are dealing with a preplanned provocation.”

He said Russia was calling for a “professional and fair investigation” and, he said, “all of this looks like an attempt at all costs to create a pretext for demanding that the U.N. Security Council side with opponents of the regime and undermine the chances of convening the Geneva conference.”

And in a final note aimed clearly at the United States, Mr. Lukashevich added, “We are urging all those who have the ability to influence the armed extremists to make every effort to put an end to provocations with the use of poisonous chemical weapons.”

At least one photograph posted on Facebook by an activist showed what looked like a makeshift rocket from the assault. But loyalist militias and Hezbollah have both fired many types of makeshift rockets at rebel positions and neighborhoods in this war, and could presumably be suspects for any attacks with improvised rockets on a rebel-controlled neighborhood.

Syria’s state news service, SANA, denied that the Syrian government forces had used chemical weapons and accused news outlets reporting the allegations of being “partners in the shedding of Syrian blood and supporting terrorism.” It also said the reports sought to “divert the special committee for the investigation of chemical weapons from carrying out its mission.”

The Syrian ambassador to Russia, Riyad Haddad, also told the Interfax news agency that the reports were false.

“The Syrian forces have never used, do not use and will not use chemical weapons even if they had them,” Mr. Haddad said. “All reports on this topic are aimed at repeating the Iraqi scenario in Syria, where there were allegedly weapons of mass destruction.”

Argentina, which currently holds the rotating presidency of the United Nations Security Council, called an urgent meeting for later Wednesday over the reports. But it was far from clear whether any action would be taken by the council, which has been riven by discord over Syria since the conflict there began.

An activist reached via Skype in Erbeen said the attack began at 2 a.m., when rockets struck surrounding areas. He and his colleagues rushed to evacuate the wounded, and they had to break down doors of homes to get them out.

“I saw many children lying on beds as if they were sleeping, but unfortunately they were dead,” said the activist, who gave his name as Abu Yassin. He added that he believed that the number of dead was in the hundreds.

“We thought this regime would not use chemical weapons, at least these days with the presence of the U.N. inspectors,” he said. “It is reckless. The regime is saying, ‘I don’t care.’”

Another activist, who gave only his first name, Mohammed, said he was in Zamalka when the rockets landed. He said he was helping to evacuate the wounded when his eyes started burning, his vision went blurry and he felt dizzy. He said the dead and wounded were taken to hospitals in a number of nearby districts, making it hard for family members to find them. In Zamalka, many of the dead were laid out in front of a mosque, where someone called over the loudspeakers for residents to come and identify their relatives.

The Syrian Observatory for Human Rights said dozens of people were killed in the attacks. The exile opposition Syrian National Council put the number of those killed at 1,300. Verifying any death toll was difficult, given that independent news organizations have little or no direct access to most of the country.

Louay Mekdad, a media coordinator for the military wing of the opposition Syrian National Council, said the attack showed that Mr. Assad no longer feared being held accountable by world powers. “Bashar al-Assad doesn’t care any longer about red lines since he has already exceeded too many of them while the world has showed no reaction,” Mr. Mekdad said. “This means the alleged lines never existed.”

He said opposition leaders had asked members of the United Nations team to visit the attack sites but they had said they were waiting for permission from the government.

He called on the United Nations Security Council and international powers to “live up to their moral and historic responsibility” to protect civilians in Syria. “If the international community doesn’t move now, when it is going to move?” he asked.

At a televised news conference, George Sabra, a leader in the coalition, accused the international community of complicity in the killings by not intervening to protect Syrians.

“American resistance is killing us,” he said at a televised news conference in Istanbul. “The silence of our friends is killing us. The betrayal of the international community is killing us. The disinterest of the Arabs and the Muslims is killing us. The hypocrisy of the world that we considered free is killing us and killing us and killing us.”

He called for increased international support for the opposition and for the rebels fighting to topple Mr. Assad.

The reported attack came nearly exactly one year after President Obama declared the use of chemical weapons a “red line” that could lead to a stronger American response. American officials in June said they believed that Mr. Assad’s forces had used chemical weapons “on a small scale” several times in the last year, but that did not lead to a substantive change to American involvement in the conflict. Russia, which has strongly backed Mr. Assad, last month accused rebel fighters of using the weapons in Khan al-Assal.

The British foreign secretary, William Hague, said in a statement that he was “deeply concerned by reports that hundreds of people, including children, have been killed in airstrikes and a chemical weapons attack on rebel-held areas near Damascus.”

“These reports are uncorroborated and we are urgently seeking more information. But it is clear that if they are verified, it would mark a shocking escalation in the use of chemical weapons in Syria,” he said, urging the Syrian government to allow the United Nations team currently in Syria “immediate access” to the location of the claimed attacks.

A spokesman for the French Foreign Ministry condemned the “murderous attacks attributed to the Syrian regime in the region of Damascus” in a statement but declined to confirm reports that those attacks involved the use of chemical weapons. The spokesman, Vincent Floreani, said France had called for an inquiry into the allegations and Foreign Minister Laurent Fabius would speak further on the situation later Wednesday.

A spokeswoman for the European Union’s top foreign policy official, Catherine Ashton, said the bloc was looking into the reports. The spokeswoman, Maja Kocijancic, added that the union did not have the facilities to verify the use of chemical weapons but that it had repeatedly deplored the humanitarian situation in Syria and urged a political solution. The European Union has also backed the work of the United Nations.