BEIRUT, Lebanon — As five warplanes circled on Monday, Marwan Habaq, trapped in a shrinking rebel-held enclave near Damascus, the Syrian capital, agonized over how best to reunite with his wife and infant daughter.

The Syrian government and its allies had managed overnight to split the enclave, eastern Ghouta, into three blocs, each surrounded and besieged, leaving Mr. Habaq suddenly separated from his family, he said by phone.

The fracturing of the enclave, one of the last major rebel-controlled areas in Syria, could be a turning point after three weeks of a relentless scorched-earth campaign by pro-government forces, backed by heavy Russian airstrikes. In the pivotal battle for the northern city of Aleppo a year and a half ago, a government advance that divided the rebel-held pocket in two signaled the beginning of the end for insurgents.

The sense of chaos has grown in recent days in eastern Ghouta. Government advances have sent residents fleeing into the remaining rebel territory, especially its main city, Douma, some accompanied by their livestock as agricultural areas were lost, and trying to squeeze into already crowded basements.