Jim Michaels

USA TODAY

WASHINGTON — The Pentagon said Friday it is unlikely that pro-Russian separatists in east Ukraine could obtain or operate the sophisticated missile system allegedly used to shoot down the Malaysia Airlines plane without Russian help.

"It strains credulity to think that they could do this without some measure of Russian support and assistance," Rear Adm. John Kirby, the Pentagon press secretary, said Friday. "It is a sophisticated system."

The Pentagon says there is strong evidence the missile, a SA-11, was fired by Russian-backed separatists since the missile was fired from an area controlled by rebels.

The international outrage generated by the tragedy is placing additional pressure on Russian President Vladimir Putin to back off support for the rebels.

"Time and again, Russia has refused to take the concrete steps necessary to de-escalate the situation," Obama said Friday.

Defense Secretary Chuck Hagel lashed out at Russia in unusually blunt language on Friday. "The Russians have been instigating this trouble in Eastern Ukraine," he said in an interview on Bloomberg Television's Political Capital with Al Hunt. "The Russians continue to isolate themselves in the world. ...This is a very serious international incident. And the Russians are going to have to take some responsibility."

Kirby said it is not clear whether the rebels thought they were targeting a Ukrainian military aircraft. A Ukrainian fighter and cargo aircraft were shot down earlier this week.

The Obama administration said preliminary evidence suggests the cargo aircraft was hit by a surface-to-air missile. Ukraine's government says the fighter was shot down by a Russian aircraft.

Kirby said there was no specific intelligence suggesting Russia provided separatists with the SA-11, but he said Russia continues to support separatists with arms, financing and training. The Pentagon said Russia has been supplying rebels with tanks and armored vehicles.

Kirby said Russia has massed between 10,000 to 12,000 troops near the border of Ukraine. "They're growing in size week by week," he said.

Samantha Powers, the U.S. ambassador to the United Nations, addressed an emergency meeting of the Security Council on Friday, saying the United States could not rule out that Russia provided technical assistance in deploying the system.

Russia's U.N. ambassador, Vitaly Churkin, did not respond to the U.S. allegations, but called for an international commission to investigate the crash, the Associated Press reported.

He asked why Ukraine allowed civilian aircraft to fly over an area where military clashes and airstrikes were taking place, according to the AP. Putin said both sides should put down their arms.

Central to the Obama administration's argument linking Russia to the attack is the type of missile allegedly used by the separatists.

Unlike portable anti-air weapons, which can fit in the trunk of a car and be fired by one person, the SA-11 is a sophisticated Russian-built system that is typically tied into a radar and requires a trained crew to operate.

The Malaysia Airlines flight was cruising at 33,000 feet when it was hit, well beyond the range of a portable weapon system, which can generally reach about 10,000 feet, according to IHS Jane's Defence Weekly.

Typically, the SA-11 is deployed in a system with separate vehicles for a command post, a radar system and missiles in launchers, according to IHS Janes. The missiles could only be operated by a trained crew, the report said.

The Ukrainian Interior Ministry released a video that they said showed a truck with an SA-11, also called a Buk, with one missile missing headed for the Russian border, according to the AP. The video could not be verified.