U.N. military observers left Damascus on Monday after a four-month mission in which they became helpless spectators of the spiraling conflict, instead of monitoring a ceasefire between Assad’s forces and rebels.

U.S. Secretary of State Hillary Clinton said earlier this month that the United States and Turkey were looking at all measures to help Syrian rebel forces, including a no-fly zone there as the conflict deepens.

Lavrov said in an interview with Sky News Arabia published on Saturday that any attempt to use humanitarian concerns as a pretext to establish no-fly zones or security zones on the ground in Syria “for military purposes” was unacceptable.

A few days after Clinton spoke, U.S. Defense Secretary Leon Panetta said a no-fly zone was not a “front-burner” issue and Turkish newspapers quoted the U.S. ambassador to Ankara as saying there were serious legal and practical obstacles to such an idea.