THE United States and other Western powers are now grasping for intelligence on the precise circumstances of the loss of the Malaysian Airlines Boeing 777 over eastern Ukraine today amid troubling, if as yet unproven, suspicions that Russia may in the end be forced to take either direct or indirect responsibility.

The diplomatic fall-out from the disaster is potentially enormous in scope and complexity but determining what it could be exactly has to await confirmation on what befell the aircraft.

If indeed it was shot down, the question of who targeted it, and with what, will become paramount.

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Terming the loss of the plane "a terrible tragedy", President Obama, who was on a previously scheduled visit to Delaware, said the US would help in every way possible to try to determine exactly what happened. 

He added that a first priority was to determine how many American passengers might have been on board. Reports said that there were 23 Americans on the passenger manifest.

The crash came just one day after Washington imposed a new layer of economic sanctions on Russia for allegedly continuing to stoke the rebellion by pro-Russian activists in eastern Ukraine.

Among US complaints is that Moscow has allowed an unimpeded flow of weaponry across the border, possibly including surface-to-air missile launchers.

Earlier, Moscow had made public its dismay at the new US sanctions, in particular, with sharp comments, including from President Vladimir Putin.

"As far as sanctions are concerned, they usually have a boomerang effect, and will lead US-Russia relations down a dead end," Mr Putin said.

"It's a great pity that our partners are following this path. But our doors are open for a negotiating process, for a way out of this situation.

Before leaving for Delaware, Mr Obama spoke directly with Mr Putin by telephone.

The call, which had been requested by Moscow, was ostensibly about the sanctions. However, officials in Moscow said Mr Putin informed President Obama about the loss of the Malaysian aircraft.

The US had already made clear that still tougher sanctions were on the table in case Russia fails to defuse the situation in Ukraine. Should any evidence emerge that the jet was hit by weaponry fired by pro-Russian forces in that country and supplied by Moscow the chances of additional measures being imposed will surely grow dramatically.

Even more serious measures would be considered, however, were it ever to be determined that it was shot down by Russian forces directly, whether intentionally or accidentally.