The United States believes the missile system was transferred into eastern Ukraine from Russia "in recent days or weeks," a senior administration official told CNN, and that the system was operational at the time.

The United States believes pro-Russian separatists could not have operated it without Russian training, the official said, noting that it's unknown whether Russian personnel were on scene when the plane was shot down.

U.S. officials believe the plane was "likely downed by a surface-to-air missile ... operated from a separatist-held location in eastern Ukraine," the U.S. ambassador to the United Nations, Samantha Power, told an emergency session of the U.N. Security Council. If pro-Russian separatists are responsible for shooting down the plane with a missile, investigators can't rule out the possibility that Russia offered help to operate the system, she said.

Power also said Russia should take steps to cool tensions in Ukraine.

"Russia can end this war," she said. "Russia must end this war.

At least one American on board

Obama confirmed that at least one U.S. citizen was aboard the plane; Quinn Lucas Schansman was a student at International Business School Hogeschool van Amsterdam, according to his Facebook page. A majority of the passengers (at least 173) were Dutch.

"No one can deny the truth that is revealed in the awful images that we all have seen, and the eyes of the world are on eastern Ukraine, and we are going to make sure that the truth is out," Obama said.

He called for an immediate cease-fire in the region and for a "credible international investigation" into what happened.

Among the evidence cited by U.S. officials and others for their conclusions was an audio recording released by Ukrainian intelligence officials which purportedly feature pro-Russian rebels and Russian military officers discussing a surface-to-air strike and the crash of a civilian jetliner.

"How are things going there," a man identified as a Russian intelligence agent asks.

"Well, we are 100% sure that it was a civilian plane," a man identified as a pro-Russian fighter responds.

"Are there a lot of people?" the Russian officer asks.

The rebel fighter then utters an obscenity and says, "The debris was falling straight into the yards."

CNN cannot confirm the authenticity of this audio, or other similar recordings.

Also, in a news conference Friday, the chief of Ukraine's security service, Valentyn Nalyvaichenko, said the Buk missile system that shot down the airliner crossed the border from Russia only "right before" the attack. He didn't say how investigators know that, however

Ukraine's Interfax news agency reported claims by an adviser to Ukrainian Minister of Internal Affairs Anton Geraschenko that the launcher, as well as the flight data recorders from MH17, were handed over to Russian agents across the border at a checkpoint in the Luhansk area overnight.

A senior Ukrainian official who spoke to CNN also accused Russia of carrying out a cover-up of its role in the shoot-down.

He cited video showing a Buk launcher being moved toward Russia overnight.

CNN could not independently confirm the claims.