What will Putin do if U.S arms Ukraine?

That prospect is increasing as the United States openly accuses its former Cold War foe of sending tanks and troops in unmarked uniforms across the border into eastern Ukraine, boosting separatists in their battle against Kiev’s beleaguered forces.

The revelations are sharpening Obama’s dilemma as his White House considers whether to send “lethal” weapons, including anti-tank and anti-mortar systems to the outgunned Ukrainian armed forces.

Obama’s nominee to become the next Defense Secretary, Ashton Carter, bolstered that idea during his confirmation hearing on Wednesday.

Carter said he was “very much inclined” to support sending what the administration calls “lethal” defensive arms to the Ukraine government.

“We need to support the Ukrainians in defending themselves,” he said.

READ: Dire scene in Ukraine

Still, Obama could face resistance on the world stage.

German Chancellor Angela Merkel, the most powerful European leader, who will meet Obama at the White House on Monday, has already made clear Germany will not supply weapons to Kiev. Merkel was traveling to Kiev and Moscow with French President Francois Hollande before coming to Washington in an effort to forge a new peace deal.

Some Russian experts believe a move to arm Ukraine could backfire, and play into Putin’s conspiratorial mindset.

Sean Kay, an expert on Russia and NATO at Ohio Weslyan University said that Putin could also swiftly increase Russian assistance to rebels in a bid to lock in quick battlefield gains. He said it would be six months to a year before a U.S. operation could have a measurable effect on the situation on the ground in Ukraine.

“The moment Putin thinks this weaponry is coming in, he could dramatically escalate things in a very dangerous way to take advantage of the current window,” he said.

CNN