U.S. looks on warily as Russian helicopters arrive in Syria

A U.S. official told CNN Wednesday that four Russian military helicopters have arrived at the airbase in Syria that Russia is beefing up, adding to the mobility of the Russian forces there.

The four helicopters also appear in satellite images provided to CNN by AllSource Analysis. The company said they were taken Tuesday and show Bassel al-Assad International Airport in Syria with newly paved areas, tanks and other equipment.

Even as Russia moves tanks, mobile artillery and armored vehicles into Syria, Defense Secretary Ashton Carter at this point is deferring to Secretary of State John Kerry to lead U.S. discussions with his Russian counterpart, Foreign Minister Sergei Lavrov, on Russian military moves into Syria, according to the Pentagon.

"Because we already have effective communications with Russia through a variety of channels, most notably between Secretary Kerry and his counterpart, to date there has been no reason for Secretary Carter to initiate another," Pentagon press secretary Peter Cook said in an emailed statement.

Kerry said Wednesday that in his third conversation with Lavrov in less than a week, he made "clear that Russia's continued support for (Bashar al-Assad) risks escalating the conflict and undermining our shared goal of fighting extremism."

What is Russia up to in Syria? 01:56

Kerry told CNN's Elise Labott that Lavrov assured him that the Russian plan is to fight ISIS, but Kerry said he isn't taking it "at face value, because we look at the type of airplanes or the types of munitions and so forth, and it obviously raises much more serious questions about what is happening."

He also said the Russians are proposing a "military-to-military conversation" aimed at ensuring Russian and coalition forces don't conflict with one another in the skies and elsewhere.

Behind the scenes, top military officials agree, and they said it is becoming essential to determine Russia's military intentions. If Russian ground forces were to begin to move around, the U.S. would want to know where those units are to ensure they are not accidentally targeted by coalition air forces, a defense official told CNN.

Similarly, if Russia moves in fighter jets, the Pentagon would need to discuss in detail with the Russians how to manage potential conflict in the airspace.

Pentagon officials said they believe the current Russian buildup will continue for several more days, but then Russia may finally signal its intentions and strategy during the upcoming U.N. General Assembly.

How to deal on a military basis with the Russians will be among the first challenges for Gen. Joseph Dunford, who takes over as chairman of the Joint Chiefs of Staff later this month. Dunford is expected to travel to the Middle East within weeks of taking office.