KIEV, Ukraine (CNN) -

International monitors investigating the Malaysia Airlines crash in eastern Ukraine said Friday the team was not given full access to the site and was greeted with hostility by armed men.

"There didn't seem to be anyone really in control," Michael Bociurkiw, spokesman for the Organization for Security and Co-operation in Europe team, told CNN's Christiane Amanpour.

Armed men, apparently pro-Russia militants, loosely guarded the area but couldn't answer the monitors' questions, he said.

Bociurkiw said the group only stayed about 75 minutes and examined about 200 meters at the scene before being forced to leave. Pieces of the airplane and bodies are spread over several kilometers.

The OSCE team arrived at the crash site near Torez in a remote section of eastern Ukraine that's controlled by pro-Russian militants battling the Ukraine government.

The United States says a surface-to-air missile, possibly fired by the militants, took down Malaysia Airlines Flight 17 on Thursday as the plane traveled from Amsterdam to Kuala Lumpur in Malaysia. All 298 people on board died. U.N. Under Secretary General Feltman said 80 of the victims were children.

Lack of access to the crash site worries U.S. officials, including Ben Rhodes, deputy national security adviser for strategic communications, who tweeted: "Monitors should be able to access the crash site of MH17. US is deeply concerned by reports that separatists are denying access."

Bociurkiw said the investigation will be difficult because the plane crashed in a difficult-to-access area in the countryside with no electricity.

"I don't think too much of the crime scene has been compromised already," Bociurkiw said. "The bodies are still there. They have not been tampered with. We actually spoke to some civilian emergency workers. They said their job was just to mark where the bodies are."

The FBI is sending two investigators to work on the case, a U.S. law enforcement official said, but the Ukraine government will be in charge of the investigation.

Obama puts focus on Russia

The location of the flight recorder has not been determined.

Ukrainian Economy and Trade Minister Pavlo Sheremeta told CNN's Richard Quest that MH17's black boxes are in Ukraine. Sheremeta would not say, though, whether the government has them.

"They are on Ukrainian territory. I don't know whether we have it or the international team has it," he said.

Earlier Friday, the adviser to the exiled governor of Dontesk told CNN's Victoria Butenko that the black boxes had been retrieved and were in rebel hands, though the location was not clear.

Russia likely bears some of the responsibility for the apparent downing of Flight 17, President Barack Obama said.

In the administration's strongest words yet on the downing of the jet, 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."

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.

He and other U.S. officials stopped short of publicly placing the responsibility on Russia, which has denied involvement in the destruction of the jetliner.

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.

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.