President Obama is considering military action against Syria that is intended to “deter and degrade” President Bashar al-Assad’s government’s ability to launch chemical weapons, but is not aimed at ousting Mr. Assad from power or forcing him to the negotiating table, administration officials said Tuesday.
A wide range of officials characterized the action under consideration as “limited,” perhaps lasting no more than one or two days. The attacks, which are expected to involve scores of Tomahawk cruise missiles launched from American destroyers in the eastern Mediterranean Sea, would not be focused on chemical weapons storage sites, which would risk an environmental and humanitarian catastrophe and could open up the sites to raids by militants, officials said.
The strikes would instead be aimed at military units that have carried out chemical attacks, the headquarters overseeing the effort and the rockets and artillery that have launched the attacks, according to the options being reviewed within the administration.
An American official said that the initial target lists included fewer than 50 sites, including air bases where Syria’s Russian-made attack helicopters are deployed. The list includes command and control centers as well as a variety of conventional military targets.
Perhaps two to three missiles would be aimed at each site, a far more limited unleashing of American military power than past air campaigns over Kosovo or Libya.
Some of the targets would be “dual use” systems, like artillery that is capable of firing chemical weapons as well as conventional rounds. Taking out those artillery batteries would degrade to some extent the government’s conventional force — but would hardly cripple Mr. Assad’s sizable military infrastructure and forces unless the air campaign went on for days or even weeks.
The goal of the operation is “not about regime change,” a State Department spokeswoman, Marie Harf, said Tuesday. Seeking to reassure the public that the United States would not be drawn into a civil war in the Middle East, and perhaps to lower expectations of what the attack might accomplish, Obama administration officials acknowledged that their action would not accomplish Mr. Obama’s repeated demand that Mr. Assad step down.
Some lawmakers have warned that the operation might turn out to be a largely symbolic strike that would leave the Assad government with the capability to mount sustained attacks against civilians with artillery, rockets, aircraft and conventional arms and would do little to reduce the violence in Syria, limit the flow of refugees or encourage Mr. Assad to negotiate seriously if a Geneva peace conference is convened.
Representative Eliot L. Engel of New York, the ranking Democrat on the House Committee on Foreign Affairs, suggested in an interview that the attacks go further than what appears to be under consideration by the administration, including strikes on the Syrian Air Force, its munitions depots and military fuel supplies to “tip the battle in favor of the insurgents.”
“We should try to help the rebels and help the people fighting Assad,” Mr. Engel said.
Frederic C. Hof, a senior fellow at the Atlantic Council who previously worked on Middle East issues for the State and Defense Departments, has urged that the Obama administration consider a broader military mission: destroying or significantly degrading the ability of the Assad government to carry out intensive artillery, aircraft and rocket attacks with conventional as well as chemical warheads on the civilian population.
“Something that is significantly less than that, something that is seen as symbolic, I think would just enable Bashar al-Assad to say I have stood up to the world’s only superpower and faced it down,” he said.
The main American attack is expected to be carried out by cruise missiles from some or all of the four Arleigh Burke-class destroyers within striking range of Syria in the Mediterranean: the Mahan, the Barry, the Gravely and the Ramage.
Each ship carries about two dozen Tomahawk cruise missiles, a low-flying, highly accurate weapon that can be launched from safe distances of up to about 1,000 miles. Tomahawks were used to open the conflicts in Afghanistan in 2001, in Iraq in 2003 and in Libya in 2011. Attack submarines also carry Tomahawks and are assumed to be on station in the Mediterranean as well.
Officials said that while Syrian rocket and artillery sites were expected to be targeted, there were no current plans to use Tomahawks to crater airfields used by the government to receive weapons and military supplies from Iran, an important lifeline for the Assad government.
Weapons experts said that Tomahawk missile strikes, while politically and psychologically significant, could have a limited tactical effect. The weapons are largely fuel and guidance systems and carry relatively small high-explosive warheads. One conventional version contains about 260 pounds of explosives and another version carries about 370 pounds. Each is less than the explosive power of a single 1,000-pound air-dropped bomb.
The weapons are not often effective against mobile targets, like missile launchers, and cannot be used to attack underground bunkers. Naval officers and attack planners concede that the elevation of the missile cannot entirely be controlled and that there is a risk of civilian casualties when they fly slightly high.
Some officials have also cautioned that Iranian-backed Hezbollah militants might step up terrorism around the region in reaction to American strikes on Syria. Another risk is that Mr. Assad might respond to the attack by firing missiles at Turkey or Jordan or mounting even more intensive attacks against civilians.
Although some experts believe that the Syrian government already has its hands full trying to contain the rebels and would not relish a war with the United States, they say that the Obama administration needs to be prepared for another round of airstrikes should Mr. Assad raise the stakes.
In an indicator of the complexities within Syria’s civil war, and the difficulties faced by the Obama administration in any effort to guide the conflict’s path, jihadi fighters opposed to Mr. Assad were warning one another to take steps to avoid being hit in any impending American attacks.
On Monday night, one prominent member of the Nusra Front, a rebel group aligned with Al Qaeda and designated a terrorist organization by the United Nations and the United States, used a Facebook posting to urge fellow members to move away from their bases or positions in Syria.
“All fighters in Jabhat al-Nusra,” he wrote, using the organization’s Arabic name, “please constantly change your positions and don’t share anything online. There is a conspiracy by America and its tails to hit our positions.”
Attacking chemical weapons storage sites comes with the same difficulties and risks associated with attacking munitions depots generally, and with its own special dangers, which the American military encountered in two wars in Iraq. First among them are risks of contamination to the very Syrian civilians that any military action would officially be intended to protect.
Many veterans suspect that some of the effects of Gulf War syndrome that afflicted veterans of the Persian Gulf war of 1991 were caused by exposure to chemical weapons released in clouds by conventional airstrikes against Iraq’s chemical weapons sites in southern Iraq.
After the first gulf war, an American Army unit near Kuwait breached chemical weapons while destroying conventional munitions at Khamisiyah, creating an environmental hazard that persisted throughout the American occupation of Iraq after the invasion in 2003.
Similarly, airstrikes in 1991 on bunkers at the Muthanna chemical weapons complex near Samarra, Iraq, led to security and environmental problems that continue to the present day.
During the Clinton administration, the United States and its NATO allies carried out extensive airstrikes against Serb forces in Bosnia, which weakened them to the point that a peace settlement to end the Bosnia war was negotiated in 1995 at an American Air Force base in Dayton, Ohio.
Similarly in Kosovo in 1999, an intensive NATO air campaign that lasted 78 days led to an agreement in which Yugoslav forces withdrew from Kosovo, and the region achieved autonomy and eventually independence.
Mr. Obama’s national security aides have been studying the NATO air war in Kosovo as a possible blueprint for acting militarily in Syria without a mandate from the United Nations.