How can U.S. respond to Russia's military presence in Syria?

As the United States and NATO watch Russia apparently beefing up its military presence in Syria, a former intelligence official said the only way the U.S. can respond is by using its influence to cut off air routes for Russian shipments, with particular focus on Iraq.

Former CIA deputy director and CBS News senior security contributor Michael Morell explained the three routes to Syria the Russians could use.

"One is to overfly Turkey. The Turks have said no. There's been a long history to that. The other way, and that's what's been happening, is over Bulgaria and Greece. But now the United States has reached out and the Bulgarians have agreed no more flights," he said Thursday on "CBS This Morning."

"The Iranians yesterday said to the Russians, you can overfly Iran, but that still means that they need to overfly Iraq. So I think the diplomatic focus is now going to be on Iraq, and the U.S. is going to put tremendous pressure on Iraq to stop those flights. We'll see what the Iraqis do," Morell said.

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Approximately 100 Russian Marines, several Russian Condor transport aircrafts and modular housing for 1,000 people arrived this week at the Assad International Airport.

"Not a lot of clarity on exactly what's happening here, but the leading theory is that the Russians are building a air base, and the concern is that they might conduct combat air operations out of that air base on behalf of President Assad and his fight against ISIS, al Nusra and the moderate opposition. So that's the concern," Morell said.