Defense Secretary Ash Carter has so far played no role in the U.S. discussions with Russia over its military build-up in Syria and the complications that could have on coalition airstrikes against Islamic State militants there.

Carter has not spoken with Russia Defense Minister Sergei Shoigu, Pentagon press secretary Peter Cook said. Instead, Secretary of State John Kerry has been calling Russia to seek information on the military activities in Syria.

It was unclear why the strategy changed from last year, when then-Defense Secretary Chuck Hagel made several calls to Shoigu to express concern over Russia's annexation of Ukraine's Crimea region.

Carter will call Shoigu when "it makes sense," Cook said.

Russia recently sent transport planes, tanks and advisers to an air base in Syria.