By Michael Pearson, Laura Smith-Spark and Nic Robertson
KIEV, Ukraine (CNN) -- Russia likely bears some of the responsibility for the apparent downing of Malaysia Airlines Flight 17, U.S. President Barack Obama indicated Friday, saying pro-Russian fighters in Ukraine couldn't have operated sophisticated surface-to-air missile batteries without Russian training and support.
In the administration's strongest words yet on the downing of the jet, which left 298 people dead, Obama said rebel fighters couldn't have operated the surface-to-air missile believed responsible for the shootdown "without sophisticated equipment and sophisticated training, and that is coming from Russia."
He and other U.S. officials stopped short of publicly placing the responsibility on Russia, which has denied any involvement in the destruction of the jetliner, whose debris lay strewn Friday across the eastern Ukraine farm field where it fell.
But a senior defense official told CNN that the "working theory" among U.S. intelligence analysts is that the Russian military supplied the Buk missile system to rebel fighters inside Ukraine.
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.
In his remarks to reporters, Obama said that he did not want to get ahead of the facts of who may have been directly involved in the airliner's destruction. But he said the United States would work hard to hold accountable those responsible for it.
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.
His comments echoed earlier statements by the U.S. ambassador to the United Nations, Samantha Power, who pointed the finger Friday at pro-Russian rebels. The plane went down near the town of Torez, in rebel territory in the Donetsk region.
Power told an emergency meeting of the U.N. Security Council that the plane was "likely downed by a surface-to-air missile ... operated from a separatist-held location in eastern Ukraine." 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.
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 towards Russia overnight.
CNN could not independently confirm the claims.
Monitors visit crash site
A group of monitors from the Organization for Security and Cooperation in Europe arrived at the crash site Friday and inspected a portion of it, spokeswoman Natacha Rajakovic said from Vienna.
The 21-member team reported shots being fired into the air at the crash site, which she said was "not as secured as it should be."
The United States was sending at least one FBI agent and and at least one investigator from the National Transportation Safety Board.
Tensions have been high between Ukraine and Russia since street protests forced former pro-Moscow President Viktor Yanukovych from power in February. Russia subsequently annexed Ukraine's southeastern Crimea region, and a pro-Russian separatist rebellion has been raging in Ukraine's eastern Luhansk and Donetsk regions.
Ukraine's government has accused Russia of allowing weapons and military equipment, including tanks, to cross the border illegally into the hands of pro-Russian rebels.
While Ukrainian officials implicated pro-Russian fighters and their Russian backers for the jetliner's downing, Moscow argued Ukraine was to blame.
"With regard to the claims raised by Kiev, that it was almost us who did it: In fact I haven't heard any truthful statements from Kiev over the past few months," Russian Foreign Minister Sergey Lavrov said in an exclusive interview with the state-run Russia 24 TV channel.
European Union leaders agreed this week to expand sanctions against individuals and entities in response to Russia's actions in Ukraine, with details to be decided by the end of the month. Expanded U.S. sanctions were also announced in Washington.
The Ukrainian Ministry of Infrastructure announced Friday that the airspace over Donetsk, Luhansk and part of Kharkiv where rebels are operating had been closed indefinitely.
Malaysian Transport Minister Liow Tiong Lai defended the routing of the Malaysia Airlines plane over the region, saying other carriers were sending their aircraft through the same airspace.
Three months ago, the U.S. Federal Aviation Administration prohibited U.S. airlines from flying in areas some way south of where Flight 17 crashed Thursday. Thursday night, the FAA expanded the flight restrictions to all of eastern Ukraine.
Thursday's crash marks the second time this year that Malaysia Airlines has faced an incident involving a downed plane.
In March, Malaysia Airlines Flight 370 disappeared with 239 people on board. Searchers have found no trace of the Boeing 777 or its passengers despite extensive search efforts.
Flight 370 probably flew into the southern Indian Ocean on autopilot with an unresponsive crew, Australian authorities said last month. A new underwater search is expected to begin in August.
Michael Pearson reported and wrote from Atlanta and Laura Smith-Spark wrote and reported from London. Nic Robertson reported from Kiev. CNN's Barbara Starr, Jim Sciutto, Catherine E. Shoichet, Dana Ford, Saima Mohsin, Mitra Mobasherat and Ben Brumfield also contributed to this report, as did journalist Victoria Butenko.
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