The United States has gathered a significant body of evidence that Ukrainian separatists have been trained on Russian territory in recent weeks to fire antiaircraft missiles, according to American military and intelligence officials who have raised alarms over the reports.

Among other weapons, U.S. officials said, the separatists have been trained in using mobile antiaircraft batteries — missile systems that could be moved around on vehicles and are thought to have been used in the downing of a Malaysia Airlines jet Thursday.

Although U.S. officials on Friday stopped short of assigning direct blame for the shooting down of the jet, President Obama said “evidence indicates that the plane was shot down by a surface-to-air missile” launched from rebel-held territory in Ukraine. He said the separatists lacked the ability to shoot down large planes “without sophisticated equipment and sophisticated training, and that is coming from Russia.”

The Ukrainian rebels are known to have been armed with shoulder-fired missiles that can be used against helicopters and other aircraft operating closer to the ground. But U.S. officials have worried that the more powerful, radar-guided missiles could give the separatists a new level of firepower.

Air Force Gen. Philip M. Breedlove, the U.S. commander of NATO forces in Europe, said late last month that there was clear evidence that Ukrainian rebels were being trained on Russian territory on how to operate “vehicle-borne” antiaircraft batteries.

Aviation history is littered with civilian planes that were shot from the sky, intentionally or not, by military weapons. Malaysia Flight 17 was cruising at 33,000 feet, more than half a mile higher than Mt. Everest, when a missile hit it July 17. And the missile’s range is believed to be more than twice that high.

“We have not seen any of the air-defense vehicles across the border yet, but we’ve seen them training in the western part of Russia,” he said at a Pentagon news conference.

Two weeks later, a Ukrainian An-26 military cargo plane was hit by a surface-to-air missile while flying at 21,000 feet — an attack that U.S. and Ukrainian government officials blamed on the Russian-backed rebels. Malaysia Airlines Flight 17, with nearly 300 civilians aboard, was struck by a missile at an altitude of 33,000 feet, according to U.S. and Ukrainian officials.

On Friday, U.S. officials said a preliminary intelligence assessment indicated the airliner was blown up by an SA-11 surface-to-air missile fired by the separatists.

Samantha Power, the U.S. ambassador to the United Nations, noted that the SA-11 is an advanced weapon whose use requires special training. “We cannot rule out Russian technical assistance,” she told the U.N. Security Council.

Rear Adm. John Kirby, the Pentagon press secretary, said defense officials could not point to specific evidence that an SA-11 surface-to-air missile system had been transported from Russia into eastern Ukraine. But he said it was clear the rebels had wanted to add such a weapon to their arsenal.

“We certainly knew this was a capability they aspired to have access to,” he said.

Kirby said U.S. officials had “very strong evidence” that the Malaysia Airlines flight was struck by an SA-11 missile fired from a spot along the Russia-Ukraine border but that “who pulled the trigger” remained unclear. He noted that Russian military advisers and mercenaries have been helping the rebels on Ukrainian territory.

“Whether it was a Russian military unit that did it or a separatist unit that did it, we just don’t know,” he said.

Russian officials and the separatists denied responsibility for the disaster. Rebel leaders said they lacked the capability to shoot down a plane flying so high. But in her speech at the United Nations, Power said separatists were “spotted hours before the incident with an SA-11 system at a location close to the site where the plane came down.”

She also noted that some separatists had initially posted messages online claiming responsibility for shooting down what they thought was a military transport plane but later deleted those messages.

The Ukrainian government has also released transcripts of what it said were intercepted phone conversations between separatist rebels and Russian intelligence officials that implicated them in the shoot-down.

In addition, the Ukrainian interior ministry released a video purporting to show rebels moving an SA-11 missile battery out of eastern Ukraine and into Russia. The launcher appeared to be missing one of its missiles. The authenticity of the video could not be independently verified.

James O. Poss, a retired Air Force major general, said the SA-11 was a sophisticated antiaircraft system that relies on advanced data links to coordinate its radar and missile launcher. “It would require training and fairly skilled operators to use,” he said.

The entire system can be moved from site to site on large vehicles, but its size makes it difficult to conceal. Several amateur videos purporting to show the SA-11 in eastern Ukraine have been posted on the Internet.

Poss said the antiaircraft battery would require regular maintenance and a steady supply of replacement parts, a level of servicing that the rebels would be unlikely to provide on their own. “You’re not going to drive that kind of thing around the roads of eastern Ukraine for months without needing spare parts from Russia,” he said.