Syrian troops and rebels fought over the country's biggest city Aleppo as President Bashar al-Assad's key foreign backer Iran gathered ministers from like-minded states for talks on Thursday about how to end the conflict.

Assad's troops assaulted rebel strongholds in Aleppo on Wednesday in one of their biggest ground attacks since rebels seized chunks of Syria's biggest city three weeks ago. Late in the day, each side gave conflicting accounts of how they stood.

Assad must win the battle for Aleppo if he is to reassert his authority nationwide, although diverting military forces for an offensive to regain control there has already allowed rebels to seize large swathes of countryside in the north.

Though sympathetic to the rebels, Western powers, Turkey and Sunni Muslim Arab states have not intervened militarily. Russia has given Assad diplomatic backing which has blocked U.N. action against him, while Iran has tried to bolster a key ally in an Arab world where many view non-Arab, Shi'ite Iran as a menace.

Tehran hosts a foreign ministers' conference on Thursday on Syria, but the attendees remain unknown, and Iran's latest diplomatic foray into the crisis has been met with deep skepticism by Western nations.