WASHINGTON The United States is confident that the Malaysian Airlines plane that crashed in eastern Ukraine was downed by an SA-11 missile likely fired by pro-Russian separatists most plausibly by mistake but there still is no hard evidence tying the rebels or Russia to the disaster, senior U.S. intelligence officials said Tuesday.

"Two things we don't know is who exactly pulled the trigger: We don't know a name, we don't know a rank and we're not even 100 percent sure of a nationality," said one senior U.S. intelligence official. "We don't even know why. The most plausible explanation is that it was a mistake."

The officials spoke at a briefing for journalists approved by the Obama administration in an apparent bid to keep international heat on Russian President Vladimir Putin for backing the rebels fighting to break two self-proclaimed independent republics away from Ukraine. The briefing also was aimed at discrediting explanations for the crash some bordering on the absurd being disseminated by the Kremlin and Russian news media.

Russia "created the conditions" for the deaths of the 298 people flying on the Boeing 777 from Amsterdam to Kuala Lumpur by training the rebels and providing them with tanks, rocket launchers and other weaponry, said the senior U.S. intelligence official.

Deputy national security adviser Ben Rhodes appeared more categorical.

"We do think President Putin and the Russian government bears responsibility for the support they provided to these separatists, the arms they provided to these separatists, the training they provided as well and the general unstable environment in eastern Ukraine," Rhodes told CNN in an interview.

The Kremlin has continued sending military hardware to the separatists since the July 17 crash, he said.

Russia stepped up the shipments after Ukrainian forces began recapturing territory from the rebels earlier this month, the U.S. intelligence officials said.

Ukrainian government claims that the latest shipment of 20 military vehicles took place on Tuesday "tracks with our technical analysis," said the senior U.S. intelligence official. That phrase usually is a euphemism for communications intercepts and U.S. spy satellite imagery.

The briefing came as Ukrainian forces pressed their offensive against the separatists in the self-declared Donetsk and Luhansk republics. In Brussels, European Union foreign ministers agreed to expand sanctions to Putin's inner circle and the Russian financial sector, but they delayed imposing them because of internal difference on the specifics.

The briefing largely reviewed posts made on English- and Russian-language social media after the Malaysian jetliner fell into fields near the rebel-held town of Snizhne, located in the Donetsk region near the border with Russia.

The U.S. intelligence officials repeated the U.S. assessment that flight MH17 was hit by a SA-11 surface-to-air missile fired from a mobile launcher known as a BUK that was positioned in a separatist-held area of Donetsk.

The assessment is based on a number of factors, they said, including the detection by U.S. satellites, sensors and other means of the launch of the missile at the time the airliner disappeared from radar screens.

Videos and photographs posted on social media the same day showed a BUK mobile launcher with one of its four SA-11s gone being hauled aboard a truck through the same area back to Russia, they noted, although those materials haven't yet been authenticated.

An initial review of news media photographs of the plane's wreckage "appears consistent with the penetration of rapidly moving objects, such as shrapnel from a SAM (surface-to-air missile) warhead," said the first senior U.S. intelligence official.

The rebels have received training on air defense systems and other weaponry at a huge military base outside the southeastern Russian city of Rostov, and they've shot down more than a dozen Ukrainian aircraft in recent months with missiles, although none were SA-11s, the officials said.

"Putin has had tied directly with the separatists," said a second senior U.S. intelligence official. He noted that the Russian president has referred to the rebels as "peoples' militias," three of their top leaders are Russian citizens, and calls for military volunteers to fight in Ukraine are being issued across Russia.

Voice recognition experts at the CIA and the National Security Agency authenticated communications intercepts posted on YouTube by the Ukrainian government in which separatists are heard saying that they had downed an aircraft.

"After it became evident that the plane was civilian airliner, separatists deleted social media posts boasting about shooting down a plane" and being in possession of a mobile SA-11 missile launcher, said the first intelligence official.