BEIRUT — Lakhdar Brahimi, the international envoy on a mission to Damascus seeking an end to the Syrian civil war, said Thursday that a transitional government with full executive authority should be established, perhaps within months, and should rule the country until new elections could be held.

Brahimi did not say who would serve in such a government, and he offered no details about the role that Syria’s president, Bashar Assad, would play — if any. But his comments suggested that if Assad did remain in the country, he would retain none of his authority.