WASHINGTON - US intelligence officials said on Tuesday they believe pro-Russian separatists likely shot down Malaysia Airlines Flight MH17 "by mistake."

Special:Malaysian airliner crashes in Ukraine

Last week's downing of the airliner, in which all 298 people aboard were killed, has sharply deepened the Ukrainian crisis, in which separatist gunmen in the Russian-speaking east have been fighting government forces since pro-Western protesters in Kiev forced out a pro-Moscow president and Russia annexed Crimea in March.

The intelligence officials, who spoke on condition of anonymity, said the "most plausible explanation" for why the separatists fired what the United States believes was a Russian-made SA-11 surface-to-air missile at Malaysia Airlines flight MH-17 was that they had mistaken it for some other kind of aircraft.

"Five days into it (following the crash) it does appear to be a mistake," one of the officials said in a briefing for reporters.

The officials said that their conclusion was backed up by intercepted conversations of known pro-Russian separatists, whose voice prints had been verified by U.S. agencies.

The speakers initially bragged about shooting down a transport plane, but later acknowledged that they might have made a mistake, the officials said.

The downing of the airliner, in which all 298 people aboard were killed, deepened the Ukrainian crisis. Separatist gunmen in the Russian-speaking east have been fighting government forces since pro-Western protesters in Kiev forced out the pro-Moscow president and Russia annexed Crimea earlier this year.

U.S. President Barack Obama's administration has said it is convinced that Flight MH-17 was brought down on Thursday by an SA-11 missile fired from territory in Eastern Ukraine controlled by pro-Russian separatists.

It has said the assessment was backed up both by unspecified intelligence information and by extensive social media postings by both the separatists and Ukrainian government.

The intelligence officials said on Tuesday that they had reports of as many as a dozen aircraft being fired on from separatist-controlled areas during two months of fighting between the Ukrainian government and rebel forces.

Two of those were large transport planes, the officials said.

One of the officials said that until the Malaysia Airlines plane was hit, most if not all of the aircraft which had been targeted, had been flying at low altitude.

Officials said the U.S. did not know that the separatists were in possession or control of SA-11 missile systems until after the Malaysia Airlines plane was struck.

Separatist leaders have denied they brought the Malaysia Airlines plane down, and Russia has denied any involvement in the incident, and suggested the Ukrainian government was to blame.