"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 yesterday.

US officials earlier offered a cautious initial assessment of Putin's order to begin withdrawing "the main part of our military contingents from the Syrian Arab Republic" from today.

Putin launched air strikes in September followed by a massive troop deployment, turning the tide of a long and brutal war in Syrian President Bashar al-Assad's favor, rescuing his regime from the brink of collapse.

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.

Putin's announcement appeared timed to coincide with peace talks in Geneva that have been dominated by a disagreement over Assad's fate.

Russia has remained steadfast in its public support of Assad, while opposition groups, the United States and key European countries have called on Assad to go as part of a negotiated transition.

Read more: Syria peace talks to start from March 14: UN

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