A Malaysia Airlines jet was shot down by a surface-to-air missile Thursday over separatist-controlled eastern Ukraine, apparently killing all 298 people on board and dramatically raising stakes in the conflict with the Kremlin-backed forces.

U.S. intelligence sources said tracking data from spy satellites and radar confirmed that the Boeing 777 en route from Amsterdam to Kuala Lumpur was hit in midair, but they couldn't immediately determine where the missile was launched.

Ukrainian officials said that Russia had recently supplied the separatists with the mobile Buk surface-to-air launching system, and that the military had not fired any antiaircraft missiles during the conflict with the separatists, which began in mid-April. A number of Ukrainian military aircraft have been shot down by the separatists.

Although Russia denied involvement, the downing of a passenger jet flying on a heavily traveled international route could sharply increase pressure on Russian President Vladimir Putin, who is accused by the United States, its allies and the Ukrainian government of backing the separatists.

Before the loss of the passenger jet Thursday, fighting in eastern Ukraine had claimed at least 500 lives in the four months since Russia seized and annexed Ukraine's Crimean peninsula, inspiring the separatists now occupying much of Ukraine's Donetsk and Luhansk regions.

On Wednesday, the Obama administration stepped up sanctions against Russia for supporting the separatists, targeting some of the country's largest financial institutions, energy companies and weapons firms.

The loss of another plane dealt a new blow to Malaysia Airlines. It hasn't solved the mystery of what happened to Flight 370, which disappeared on its way from Kuala Lumpur to Beijing in March.

Ukrainian Security Service communications intercepts made public after the early evening crash of Flight 17 suggest coordination between the separatists and Russian fighters across the border.

Malaysia Airlines said it lost contact with Flight 17, carrying 283 passengers and 15 crew members, after the aircraft failed to confirm its entry into Russian airspace on schedule at 5:20 p.m. The last contact was recorded about 4:20 p.m. by air traffic controllers.

The plane “disappeared from radar at [33,000 feet] and then crashed near the city of Shakhtersk in Ukraine's Donetsk region,” the Interfax-Ukraine news agency reported.

Witnesses in the militant-held crash area about 25 miles west of the Russian border described a gut-wrenching scene.

“Distorted bodies of dead people and body parts are lying around everywhere in an open field,” Sergei Kavtaradze, a member of the Security Council of the separatists' self-proclaimed Donetsk People's Republic, said in a phone interview.

Two things were clear from rescue workers' reports, he said: Everyone on board was killed, and it “was not us who shot down the plane because we don't have this hardware.”

Ukrainian officials disputed that assertion. Anton Gerashchenko, an advisor to the Interior Ministry, said Russia had recently supplied mobile missile systems to the separatists that are capable of taking down a plane at that altitude.

He demanded that Putin face an international tribunal in the disaster.

Neither the Kremlin nor major Russian media responded to the accusations that Moscow was behind the jet's downing. But Putin told his economic advisors meeting early Friday that the disaster was Ukraine's fault because of its offensive against separatists.

“This tragedy would not have happened if there were peace on this land, if the military actions had not been renewed in southeastern Ukraine. And, certainly, the state over whose territory this occurred bears responsibility for this awful tragedy,” the Kremlin news service quoted him as saying.

Ukrainian officials earlier Thursday accused Russian forces of being behind the downing the day before of a Ukrainian SU-25 ground-support warplane near Donetsk.

Shortly after the airliner went down, the former Russian special forces officer commanding the separatists in eastern Ukraine claimed responsibility for shooting down an AN-26 Ukrainian military transport plane.

“We warned them not to fly in our skies,” Igor Strelkov said on his Facebook page. “Peaceful people didn't suffer.”