Story highlights World powers have yet to even meet to negotiate details of the ceasefire

Russian airstrikes in northern Syria drew sharp criticism by Washington

Washington (CNN) With a Syrian ceasefire set to take effect Friday, the U.S. called on Russia to "put up or shut up" about implementing the truce.

The State Department acknowledged Tuesday, however, that world powers have yet to even meet to negotiate details.

"I can't say categorically that a week from last Thursday there must be a cessation of hostilities, but certainly we're going to expect that there is progress," State Department Deputy Spokesman Mark Toner said.

Russian and regime airstrikes against hospitals and schools in northern Syria drew sharp criticism by Washington and cast further doubt on the viability of the "cessation of hostilities" announced last week in Munich, scheduled to start Friday.

The Russian Ministry of Defense denied any responsibility for the attacks in a series of tweets on Tuesday, but Toner blamed the Russians for continuing to supply the Syrian regime and condemned the attacks as "unacceptable."

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