BEIRUT — Clashes between government troops and rebels raged Thursday in opposition bastions of the besieged city of Aleppo as President Bashar Assad's key state backer Iran prepared to host a gathering of allies for talks on how to end the conflict.

The regime pressed its new assault on Aleppo, Syria's largest city and commercial hub, for a second day. But blistering attacks on rebel positions from the ground and the air appear to be only slowly chipping away at the opposition's grip on its strongholds.

The state news agency claimed Wednesday that Assad's force had regained control of the Salaheddine neighborhood, the main rebel area in Aleppo. But activists said rebels were still putting up a fight there on Thursday.