U.S. intelligence agencies have concluded that a Malaysia Airlines jet flying over eastern Ukraine was brought down by a surface-to-air missile, according to a senior U.S. official.

It remains uncertain who fired the missile that shot down the Boeing 777 carrying 298 passengers and crew members Thursday, the official said. All aboard are believed to have perished.

Intelligence agencies were able to confirm a missile was fired using tracking data from spy satellites and radar, another U.S. official said. They are still analyzing the data to determine the location of the launch.

Wreckage of the plane, which was flying from Amsterdam to Kuala Lumpur, Malaysia, was scattered over a wide area of eastern Ukraine, where attacks on military aircraft by Russia-backed separatists have intensified.

Sen. Dianne Feinstein (D-Calif.), head of the Senate intelligence committee, told reporters after an intelligence agency briefing that Russian separatists had shot down more than a dozen planes and helicopters in Ukraine in recent months.

“Even before today's events … it was clear that Russia was escalating its support for the separatists, which is obviously very concerning,” she said, adding that she was awaiting further evidence on what brought down the Malaysian jet.

“It's too soon to make any conclusion about the Malaysia Airlines crash right now,” she said. “We hope to have more information within the next day or so. If evidence emerges that Russia was involved, that would obviously be extremely concerning.”

Intelligence agencies have not determined whether the missile was fired from militant-held territory in eastern Ukraine, the second U.S. official said.

Ukraine's military also has surface-to-air missiles capable of hitting an airliner flying at a high altitude.

Ukrainian officials on Thursday denied responsibility for shooting down the jet, as did militant groups.

U.S. officials said that any decisions about what steps, if any, the Obama administration would take in response to the downing of the plane would come only after they had a clearer picture of who was responsible.

If the White House concludes that it was Russia-backed militants who brought down the civilian airliner, that could force President Obama and U.S. allies in Europe to respond with new steps soon after imposing new sanctions this week on Russian financial institutions, energy companies and arms firms for Moscow’s intervention in Ukraine.

The U.S. has moved a small number of troops, fighter aircraft and ships into the region since the crisis between Russia and Ukraine began in March.

The State Department was trying to confirm reports that 23 Americans were aboard the downed aircraft, spokeswoman Jennifer Psaki said.

Obama called the crash a “terrible tragedy” and talked about it on the phone with Russian President Vladimir Putin.

White House Press Secretary Josh Earnest said Obama and Putin spoke by phone Thursday morning, at Moscow's request, to discuss the new sanctions.

News of the plane broke as they were on the phone and Putin brought up the crash during the conversation, Earnest said.

Earnest said Obama was subsequently briefed about the crash and instructed his staff to keep him updated and to contact senior Ukrainian officials to offer assistance.

Later in the day, en route to fundraisers in New York, President Obama placed separate phone calls to Ukraine President Petro Poroshenko and Malaysian Prime Minister Najib Razak, the White House said.

Times staff writer Kathleen Hennessey contributed to this report.

Twitter: @DavidCloudLAT