KIEV, Ukraine (KABC) --A Malaysia Airlines plane carrying 298 people crashed near the Russia-Ukraine border on Thursday after being shot down by a missile.
A U.S. official said they believe that a complex surface-to-air missile brought down the jet, but it's unclear who fired it.
The plane went down near a village called Hrabove, which is currently under the control of pro-Russia separatists, according to the Donetsk region government. The region has seen severe fighting between the two sides in recent days.
The passenger plane, Flight MH17 from Amsterdam headed to Kuala Lumpur, was carrying 283 passengers and 15 crew members. Officials initially thought there were 280 passengers, but they had not accounted for three infants also on the plane. Ukrainian authorities believe everyone aboard is dead, with bodies and debris spread across a 10-mile area. It is not yet known if any Americans were on the plane.
PHOTOS: Malaysia Airlines Flight MH17 crashes
The nationalities of the victims have been determined as follows: 15 Malaysian crew members, 154 Dutch, 27 Australians, 23 Malaysians, 6 British, 11 Indonesians, 4 Belgians, 1 Canadian, 3 Filipinos, 4 Germans and at least 4 French. The nationalities of 43 victims have not yet been determined.
Victoria Premier Denis Napthine told reporters in Melbourne, Australia, that a "substantial number" of world-renowned AIDS researchers and activists were among the passengers. Many were en route to next week's 20th International AIDS conference.
The Ukrainian Foreign Ministry said in a statement that it believed a Russian missile shot down the plane.
"The plane was shot down because the Russian air defense systems was affording protection to Russian mercenaries and terrorists in this area. Ukraine will present the evidence of Russian military involvement into the Boeing crash," the statement read.
The plane was flying at an altitude of 33,000 feet when it went down in Eastern Ukraine. Pro-Russia separatists denied shooting down a plane, and Ukrainian President Petro Poroshenko also stressed in a statement that the Armed Forces of Ukraine did not shoot at any airborne targets.
On behalf of the state, Poroshenko called the incident an act of terrorism and expressed "deep and sincere condolences to the families and relatives of the victims of this terrible tragedy." Russian President Vladimir Putin also tweeted his condolences to Malaysian Prime Minister Najib Razak.
President Barack Obama was briefed on the situation and has spoken to Putin about it. The White House confirmed that Putin noted the early reports of a downed passenger jet near the end of a phone call with Mr. Obama.
"It looks like it might be a terrible tragedy," the president said. "Right now we're working to determine whether there were American citizens on board. That is our first priority."
Speaking at an event in Detroit, Vice President Joe Biden said the plane had "been shot down, not an accident. Blown out of the sky."
FBI and NTSB officials will head to Ukraine in an "advisory role" in the investigation, a senior administration official told ABC News.
On Wednesday night, a Ukrainian fighter jet was shot down by an air-to-air missile from a Russian plane, Ukrainian authorities said. The pilot of the Sukhoi-25 jet was forced to bail after his jet was shot down.
The Ukrainian Defense Ministry said a second jet was hit by a portable surface-to-air missile, but added the pilot was unscathed and managed to land his plane safely. Pro-Russia rebels claimed responsibility for the two strikes.
Thursday's plane crash comes a day after Mr. Obama levied broad economic sanctions on Russia as punishment for its threatening moves in Ukraine.
In April, the U.S. government issued an order to all American air carriers barring them from traveling in the airspace over Crimea and part of the Black Sea.
This is the second Malaysia Airlines plane incident this year. Flight MH370 vanished on March 8 with 239 people on board after it took off from Kuala Lampur for Beijing.
Malaysian officials said the plane disappeared in the Southern Indian Ocean, but no wreckage has been located.
ABC News and The Associated Press contributed to this report.