GENEVA—The Obama administration took two steps back from its push for a prompt attack on Syria, allowing several weeks more for diplomacy on eliminating Syrian chemical weapons and acknowledging that an eventual United Nations resolution wouldn't threaten force, senior administration officials said.

The reversals on Friday—after a week that began with President Barack Obama insisting that Congress urgently approve military action—came as U.S. officials said talks with Russia were producing a road map for shutting down Syria's...