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Republicans on the House Armed Services Committee on Wednesday strongly urged President Obama to take a firmer stance against Russia in light of the Ukraine crisis and amid increasing disquiet on Capitol Hill about what Moscow's next move might be.

The letter, authored by eight congressmen, comes after U.S. and European security agencies estimated that Russia has deployed military and militia units totaling more than 30,000 people along its border with eastern Ukraine, Reuters reported.

The letter's authors say they are "gravely concerned" about the reported troop buildup, and urges the Obama administration to work with NATO allies to share intelligence with the Ukrainian government so they can prepare for any further incursions by Russia. They also claim the presence of Russian special forces formenting civil unrest in eastern Ukraine, and the presence of Russian naval forces and coastal troops in the Baltic.

"There is deep apprehension that Moscow may invade eastern and southern Ukraine, pressing west to Transdniestria, and also seek land grabs in the Baltics," the letter says.

In a statement accompanying the letter's release, committee chairman Rep. Buck McKeon, R-Calif., says Russia's "war on Ukraine has already started."

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    “It is time to stop speculating about possibility, and start dealing with reality," McKeon said. "Continued inaction by the President in the face of Mr. Putin’s invasion will make further Russian aggression more – not less – likely. Any show of resolve from the White House will have my full support.”

    Fox has learned that that NATO’s Supreme Allied Commander Gen. Philip Breedlove will be on Capitol Hill Thursday to speak to lawmakers from both houses of Congress in closed meetings about the gravity of the situation. Over the weekend, Breedlove raised the possibility that Moscow would move to expand its territory by annexing Transdniestria, a breakaway state whose 1990 claim of independence from the former Soviet republic of Moldova has gone unrecognized by the rest of the world.

    “I can’t tell you how awful this is,” said one congressional source who spoke to Fox on the conditition of anonymity.

    “It’s remarkable concern,” said another source. “There are senior people here are more concerned than I have ever seen them.” A third source said that information received on Capitol Hill Wednesday "reflects a deteriorating situation which prompted very serious concern."

    Russia accused Ukrainian officials on Wednesday of barring Russian commercial airline crews from going outside their planes in Ukrainian airports. Ukraine denied the allegation.

    Tensions between Moscow and Kiev have been rising since Russian forces occupied the Crimean Peninsula and stayed there throughout the referendum earlier this month where voters overwhelmingly supported Russia's annexation.

    Russian authorities previously complained that Ukrainian border guards have singled out Russian men at the frontier and blocked their crossing, fearing that they may be activists coming to stir up unrest.

    Russia's foreign ministry said in a statement that Ukrainian border guards have been forcing cabin crews of Aeroflot, the state-controlled Russian airline, to stay inside their planes. The ministry said the decision violates international law and ultimately "poses a threat to the safety of civil aviation" because the crews cannot rest properly.

    The ministry said that it had sent protest notes twice to the Ukrainian Embassy in Moscow.

    Oleg Slobodian, spokesman for the Ukrainian border service, denied any policy to keep Aeroflot crews on their planes. The only time a crew member was prevented from entry was on March 24, he said, when a passport check revealed that the man had a travel ban. In solidarity, the remainder of the crew remained on board with him.

    Ukrainian soldiers on Tuesday began their westward journey from Crimea after Ukraine's fledging government ordered the withdrawal of its troops from the Black Sea peninsula.

    Fox News' Chad Pergram, Greg Palkot and The Associated Press contributed to this report.