An armed pro-Russian separatist stands at a site of the plane crash in the Donetsk region of eastern Ukraine. 
A Malaysia Airlines passenger plane was reportedly shot down, killing everyone aboard, in Ukraine near the Russian border Thursday.

Ukrainian President Petro Poroshenko called it a “terrorist act,” reported Interfax.

Anton Gerashenko, an adviser to Ukraine’s Interior Minister, said on Facebook that the Boeing 777-200ER was flying at an altitude of 33,000 feet when it was hit by a missile fired from a Buk launcher, according to the Ukrainian news agency.

A Reuters reporter said that 23 U.S. citizens died in the crash. President Obama said U.S. authorities are “working to determine” if Americans were aboard the flight.

A witness captured footage of the fire that erupted after the plane hit the ground.
Taking the stage at 2:10 pm, President Obama said “the world is watching reports of a downed passenger jet near the Russia-Ukraine border”

“It looks like might be a terrible tragedy,” he said, adding that his team is trying to determine whether there were any Americans onboard the plane. “That is our first priority.”

“The United States will offer any assistance we can to help determine what happened, and why,” he said.

Reports show the flight was heading from Amsterdam to Kuala Lumpur.
A man works at putting out a fire at the site of a Malaysia Airlines Boeing 777 plane crash.
Dark clouds of smoke rise from a field where Malaysian Airlines 777 was shot down Thursday.

A missile from a Buk launcher, like the one pictured, hit the plane when it was at 33,000 feet, according to Anton Gerashenko, an adviser to Ukraine’s Interior Minister.
The plane that crashed in Ukraine was reportedly a Boeing 777.
The aircraft came down just 20 miles short of entering Russian airspace near the city of Donetsk, in an area where the Ukrainian government has been fighting pro-Russian rebels.

Donetsk is a stronghold for the pro-Russian separatists.