MOSCOW—Efforts to broker a cease-fire in Syria’s war and revive talks to end the five-year-old conflict faltered Friday, after a working group charged with hammering out details of a truce failed to meet.

Under an agreement reached last week by U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov, a “cessation of hostilities” was to begin on Friday. An international working group was set up to work out logistics of the truce, but Russia’s Foreign Ministry spokeswoman Maria Zakharova said Friday’s meeting...