The White House said Presidents Barack Obama and Vladimir Putin discussed Russia's planned military drawdown in Syria on Monday, hours after a shock announcement that could signal a new phase in the five-year-old conflict.
"They discussed President Putin's announcement today of a partial withdrawal of Russian forces from Syria and next steps required to fully implement the cessation of hostilities," the White House said in a statement.
US officials offered a cautious initial assessment of Putin's decision to withdraw "the main part" of its military contingent from Tuesday.
"At this point, we are going to see how things play out over the next few days," a senior administration official told AFP.
Putin launched air strikes against rebel positions in September followed by a massive troop deployment.
That turned the tide of a long and brutal war in Syrian President Bashar al-Assad's favor, rescuing his regime from the brink of collapse.
Putin's announcement appeared timed to coincide with peace talks in Geneva that have been dominated by a disagreement over Assad's fate.
The timing and the suddenness of the announcement will raise questions about whether Russia remains steadfast in its support for Assad.
Moscow has long refused calls from opposition groups, the United States and key European countries for Assad to go as part of a negotiated transition.
"A political transition is required to end the violence in Syria," Obama said.
A recent "cessation of hostilities" has been frequently breached but, Obama said, led to a "much-needed reduction in violence."
The White House sought to turn the screws on Assad, just as his backing from Russia was called into question.
"Continuing offensive actions by Syrian regime forces risk undermining both the Cessation of Hostilities and the UN-led political process," the White House cited Obama as saying.
"The president also noted some progress on humanitarian assistance efforts in Syria, but emphasized the need for regime forces to allow unimpeded access for humanitarian assistance delivery to the agreed-upon locations, notably Daraya."
AFP contributed to this report