This story has been updated with US intelligence report that a surface-to-air missile was fired at the airliner.

The crash of a Malaysian Airlines jet over Ukraine—possibly shot down by a missile—threatens Russia at a time it is already tottering economically and diplomatically.

At this stage, conclusions as to the reason for the crash, which killed 295 people, are conjecture. Just four months after another Malaysian airliner vanished without a trace, no one is likely able to credibly assert yet what happened today over Donetsk. But fingers are pointing primarily at the Moscow-backed separatists, who have shot down several Ukraine government planes in recent months, including one they may have downed yesterday.

This is why the crash places Putin—the separatists’ idol and suspected patron—in a crisis of a gravity he has rarely faced. If it turns out that the separatists shot down the passenger jet, the incident almost certainly will become a tipping point in marking them as terrorists and not mere rebels. To the degree he continues to support them, Putin himself risks shifting to dangerous new diplomatic terrain and harsh new sanctions by a West united against him to a degree it has been at no time since the Cold War. He will be seen as backing an indefensible rogue element.

Kenneth Yalowitz, a retired senior US diplomat who served several tours in the former Soviet Union, told Quartz in an email exchange:

“It is early days in determining responsibility for this disaster. But, yes, if the separatists did this using a Russian missile, or the Russians themselves did it, there will be serious diplomatic consequences. This would likely convince the doubters in the EU to move forward with tougher sanctions against Russia and it could lead to tougher US sanctions. Russia’s deniability of direct support for the rebels would be demolished, making Putin’s international position more difficult.”

Reports are that the passengers included some 55 Dutch, 23 Americans, and nine Britons. Matthew Rojanski, director of the Kennan Institute in Washington, told Quartz that new sanctions are certain. He said:

“Still not enough hard info, but this does seem like a sophisticated weapons system and an intentional downing (even if a case of mistaken identity—i.e. the rebels thought it was a Ukrainian transport plane). This is going to have massive repercussions for Russia-West relations, probably yielding a new round of EU sanctions very soon.”

On Thursday afternoon, US intelligence agencies reported that a surface-to-air missile was fired at the Malaysian airliner. Who fired the missile was not clear. But the report confirms that the airliner was shot down and did not crash from mechanical failure.