American intelligence authorities believe a Malaysia Airlines passenger jet carrying 298 people on board was shot down with a surface-to-air missile Thursday as it flew over eastern Ukraine at about 30,000 feet, a U.S. official said.

The official said the U.S. was still working to determine additional details about the crash, including who fired the missile and whether it came from the Russian or Ukraine side of the border. But the official said it appeared unlikely the Ukrainian government forces shot down the plane because they don’t have those capabilities.

The official was not authorized to discuss the matter publicly so spoke on condition of anonymity.

Malaysia Airlines Passenger Jet Shot Down With Surface to Air Missile: U.S. Intelligence

Debris from Malaysia Airlines Flight 17 is shown smouldering in a field July 17, 2014 in Grabovo, Ukraine near the Russian border. (Pierre Crom/Getty Images)

Meanwhile, Ukraine’s security services produced what they said were audio recordings of intercepted phone calls that prove rebels were responsible for shooting the jet — Malaysia Airlines flight 17 en route to from Amsterdam to Kuala Lumpur, Malaysia — down.

Malaysian Prime Minister Najib Razak said the aircraft never made a distress call.

Malaysia Airlines Passenger Jet Shot Down With Surface to Air Missile: U.S. Intelligence

Debris from Malaysia Airlines Flight 17 is shown smouldering in a field July 17, 2014 in Grabovo, Ukraine near the Russian border. (Pierre Crom/Getty Images)

Pro-Russia rebels have denied any involvement and Russian President placed blame on Ukraine.

“Certainly, the state over whose territory this happened bears responsibility for this terrible tragedy,” Putin said at a meeting with advisers. “This tragedy would not have happened if there was peace in this land, they would not have been renewed war-like actions in the southeast of Ukraine.”

Malaysia Airlines Passenger Jet Shot Down With Surface to Air Missile: U.S. Intelligence

People walk amongst the debris, at the crash site of a passenger plane near the village of Grabovo, Ukraine, Thursday, July 17, 2014. (AP Photo/Dmitry Lovetsky)

Most of the passengers on the doomed flight were Dutch, with 154 from the Netherlands, 43 — including 15 crew and two infants — from Malaysia, 27 from Australia, 12 — including one infant — from Indonesia, nine from the U.K., four from Germany, four from Belgium, three from the Philippines, one from Canada and 41 with unconfirmed nationalities.

Fox News initially reported that the flight’s manifest listed 23 passengers with American citizenship, a number that has since been discredited by Malaysia Airlines officials.

“We are horrified by the crash of Malaysia Airlines flight MH17,” Secretary of State John Kerry said in a statement. “There are no words adequate to express our condolences to the families of the nearly 300 victims.”

“We are horrified by the crash of Malaysia Airlines flight MH17.”
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“We are reviewing whether any American citizens were aboard the flight,” he added.

President Barack Obama addressed the tragedy shortly before planned remarks on infrastructure spending Thursday afternoon.

“It looks like it may be a terrible tragedy…we’re working to determine whether or not there were American citizens on board,” President Barack Obama said. “That is our first priority.”

Malaysia Airlines Passenger Jet Shot Down With Surface to Air Missile: U.S. Intelligence

Electronic flight information board shows Malaysia Airlines flight MH 0017, after the plane crashed in eastern Ukraine at Kuala Lumpur International Airport in Sepang, Malaysia, Friday, July 18, 2014. Malaysia Airlines said it lost contact with Flight 17 over Ukrainian airspace Thursday, flying from Amsterdam to Kuala Lumpur, Malaysia. (AP Photo/Vincent Thian)

Obama called President Poroshenko of Ukraine Thursday evening to offer assistance and emphasized that all evidence from the crash site must remain in place until international investigators arrive.

However, pro-Russian separatists in eastern Ukraine said they had found MH17’s “black box.” A Russian radio station reported that the “black box” was traveling to Moscow “for investigation.”

“This is truly a grave situation,” Vice President Joe Biden said, speaking in Detroit. “It’s important we get to the bottom of this sooner than later because of the possible repercussions that can flow beyond from this, beyond the tragic loss of life.”

Kerry later added that the U.S. is willing to assist with the investigation.

“The United States Government remains prepared to assist with a credible, international investigation any way we can, and we will continue to be in touch with all relevant partners as we seek the facts of what happened today,” he said.

Editor’s note: This post has been updated with additional information, including about the number of American citizens initially reported to have been on the flight.

The Associated Press contributed to this report.

Follow Oliver Darcy (@oliverdarcy) on Twitter