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 to both combat Islamic State extremists and assist moderate rebels trying to oust Syrian President Bashar al-Assad.

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 Russian Foreign Minister Sergey Lavrov, 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.

In the afterglow of the Iran nuclear deal, which was hailed by the administration as the kind of diplomacy that can be achieved when Russia and the United States co-operate, U.S. officials had hoped for a change in Russia's position about Syria, potentially even enlisting its support to move Assad out.

Moscow's latest actions, however, have taken many by surprise and further muddied efforts to fight Islamic State of Iraq and Syria (ISIS) 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."

That history, he said, might indicate that Putin has a long-term goal, but what that might be is "not clear exactly."

Some officials believe Putin may simply be protecting Russian assets, others that he is betting on Assad's survival and still others that he may be laying the groundwork for a large Russian presence in a post-Assad Syria.

Viable options appear few and far between.

One U.S. official, speaking on condition of anonymity because he was not authorized to speak publicly, likened the menu of options to "spaghetti on the wall."

New sanctions against Russia not discounted

Longstanding calls from Turkey to create a no-fly zone in northern Syria could be revisited, the official said. However, neither the administration nor its allies are eager for any kind of direct confrontation with Russia.

Sanctions for supporting the Assad regime could also be imposed, but since such penalties have done little to deter Russian activities in Ukraine, some in the administration question their effectiveness, the official said.

State Department spokesman John Kirby held out the possibility of new sanctions on Russia but would not commit. "These actions inside Syria could very well lead to further isolation for Russia," he said.

Unease over the uncertainty led to Kerry's call on Tuesday, which came shortly after Putin defended his military assistance to Assad's government. Putin said it is impossible to defeat the Islamic State group without co-operating with Damascus and urged other countries to join the cause.

The administration's argument has been the reverse, blaming Assad for the rise of ISIS and warning that attempts to prop him up will only prolong the conflict. Last week, President Barack Obama said Putin's strategy is "doomed to fail" and the White House reinforced that message on Tuesday.

"We've made clear that further support, military or otherwise for the Assad regime is destabilizing and counterproductive, principally because Assad has lost the legitimacy to lead that country," Earnest said. "Russia's decision to double down on Assad is a losing bet."

He renewed calls for Russia to co-operate with the U.S.-led coalition conducting airstrikes to degrade and destroy ISIS in Syria and Iraq.

"The Russians indicate that they share that goal, and we'd like to see them work co-operatively with the rest of the international community to advance it," Earnest said.

Amid the uncertainty, and under criticism from some for not doing enough to help Assad's foes in the first four years of the conflict, some U.S. allies may be wavering in Putin's direction.

Last week, British Foreign Secretary Phillip Hammond told Parliament's Foreign Affairs Committee that his government was willing to accept compromises with Russia and Iran that could see Assad play a role "for some months" in a transition process.

And yet, U.S. officials categorically reject such a scenario. Kerry will discuss the matter with Hammond during a visit to London this weekend.

"Nothing's changed about the fact that we don't want to see the Assad regime getting any support," Kirby said. "There can't be a role [for] the Assad regime in efforts to stabilize the situation in Syria, much less go against [ISIS]."