Russia's recent military buildup in Syria has perplexed the Obama administration and left it in a quandary as to how to respond, complicating Washington's efforts.
Underscoring U.S. uncertainty about Russian President Vladimir Putin's intentions, Secretary of State John Kerry on Tuesday placed his third phone call in 10 days to Russia's foreign minister, seeking clarity about Moscow's moves, the State Department said. "Kerry made clear that Russia's continued support for President Assad risks exacerbating and extending the conflict, and undermining our shared goal of fighting extremism if we do not also remain focused on finding a solution to the conflict in Syria via a genuine political transition," the department said in a statement.
Moscow's latest actions, however, have taken many by surprise and further muddied efforts to fight Islamic State militants while trying to promote political transition in Syria.
In recent days Russia has sent about a half-dozen battle tanks and other weaponry - along with military advisers, technicians, security guards and portable housing units - to Syria with the apparent goal of setting up an air base near the coastal town of Latakia, a stronghold of the Syrian president.
But U.S. officials say Putin's intentions in Syria, particularly in the medium- to long-term, remain a mystery.
"The decision-making process in that country is rather opaque," White House spokesman Josh Earnest said of Russia, adding that Moscow has long used Syria as a "client state."