Former U.N. Secretary-General Kofi Annan is quitting as international peace envoy for Syria in the face of an armed rebellion against President Bashar al-Assad whose violence shows no sign of abating after 17 months of strife.

As battles raged on Thursday in Syria's second city Aleppo between rebel fighters and government forces using war planes and artillery, U.N. Secretary Ban Ki-moon announced in New York that Annan had said he would go at the end of the month.

"Kofi Annan deserves our profound admiration for the selfless way in which he has put his formidable skills and prestige to this most difficult and potentially thankless of assignments," Ban said. Talks were under way to find a successor.

Annan's mission, centered on an April ceasefire that never took hold, has looked irrelevant as fighting has intensified in Damascus and Aleppo.

A clearly frustrated Annan blamed "finger pointing and name calling" at the U.N. Security Council for his decision to quit.

In Syria, the fight for Aleppo, the latest battlefield, intensified. Rebels turned the gun of a captured tank against government forces on Thursday, shelling a military airbase used by war planes in the battle for Aleppo.