Syrian insurgents fighting loyalist forces in the northern city of Aleppo seized areas near its airport on Friday, threatening the government’s control of a strategically vital supply conduit and scoring a propaganda victory in what has become a protracted battle in Syria’s largest metropolis.

Rebel commanders reached by phone said their fighters had advanced to within a few hundred yards of the airport perimeter. Syria’s state-run media, which have portrayed the Aleppo fighting by insurgents as a futile effort by criminal gangs, inadvertently confirmed the insurgent advance, reporting that government troops deployed around the airport had repulsed attacks.

“Our fighters are in all neighborhoods close to the airport,” said a rebel commander who identified himself as a former air force pilot named Wasel. The commander, who did not provide his full name for security reasons, also said the insurgents were benefiting from replenished supplies of ammunition after chronic shortages, “which is a reason for this progress.”

The Aleppo fighting, as well as heavy clashes reported by activists in the Damascus area, came as the United Nations and the Arab League announced a successor for Kofi Annan, the special Syria envoy who resigned in frustration two weeks ago over his inability to halt the conflict, now in its 18th month. Mr. Annan’s successor, Lakhdar Brahimi, a former Algerian foreign minister and veteran diplomat who helped broker the end of Lebanon’s civil war, is expected to take up his new role in coming days.