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Russia has put a military force of 20,000 on its border with Ukraine and could easily mount an invasion of the Ukrainian mainland without warning, alarmed U.S. defense officials told NBC News on Thursday.

The buildup raised concerns that Russia, which has already taken control of the Crimean peninsula and moved to annex it from Ukraine, will escalate the conflict by invading eastern Ukraine.

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“It’s like they’re on a hair trigger,” one American defense official said.

The Russian military presence includes combat troops, heavy armor, attack helicopters, highly trained airborne forces and special-operations troops, the officials said. They stressed that there has been no change in the number or equipment in recent days.

President Barack Obama on Thursday expanded U.S. sanctions against Russian officials and threatened broad sanctions to strike at the Russian economy if Russia moves further into Ukraine.

Russia has said it has no designs on eastern Ukraine, but it has also vowed to protect the heavy population of Russian speakers there.

Defense Secretary Chuck Hagel spoke for an hour by phone with his Russian counterpart, Defense Minister Sergey Shoigu, on Thursday. Hagel told him that “because Russian forces are in control of Crimea they bear responsibility for what is happening there,” said a Pentagon spokesman, Rear Adm. John Kirby.

Pressed by Hagel, the Russian defense minister said Russian troops are on the Ukrainian border to conduct military exercises only, the Pentagon said. The two agreed to keep communications open.

— Jim Miklaszewski