Announcing Annan's resignation at the UN in New York, Ban said the UN and the Arab league were in discussions on appointing a successor to Annan.

"Mr. Annan has informed me, and the secretary general of the League of Arab States, Mr. Nabil Elaraby, of his intention not to renew his mandate when it expires on 31 August 2012."

"I wish to express my deepest gratitude to Mr. Annan for the determined and courageous efforts he has made as the joint special envoy for Syria," Ban said.

"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 assignments," Ban said.

The UN and Arab League named Annan, a Nobel Peace Prize-winning former UN chief, as its envoy on the Syria conflict on February 23. Syria's President Bashar Assad agreed a six-point peace plan with Annan but never carried it out.

Annan cites UN 'division'

Speaking in Geneva, Annan said "persistent division" within the UN Security Council had become an "obstacle to diplomacy" and said the plan commonly attributed to him was, in fact, a Security Council initiative.

'"At a time when the Syrian people desperately need action, there continues to be finger pointing and name calling in the Security Council," Annan said. "As an envoy, I can't want peace more than the protagonists."

Russia, which together with China, has vetoed past UN Security Council draft resolutions on Syria, said it regretted Annan's decision.

"We understand that that's his decision; we regret that he chose to do so," Ambassador Vitaly Churkin told reporters at the UN. Churkin insisted that Russia had always supported the envoy "very strongly."

UN draft toned down

Annan's move precedes a UN General Assembly vote due on Friday on an Arab-drafted resolution that initially called on Assad to stand down as fighting continues unabated across Syria. The latest draft has been toned down by removing the explicit call that Assad resign and that sanctions be supported.

On Thursday, Syrian rebels shelled a military air base expected to be used by regime forces to bombard the northern city of Aleppo. Meanwhile, Syrian troops have reportedly killed dozens of people in a raid on Damascus.

Syrian activists say more than 20,000 people have died since the uprising against Assad started in March 2011.

An assessment released by UN food and agriculture agencies based in Rome says 3 million Syrians affected by the fighting need food, crop and livestock assistance.

Farmers had been forced to abandon farming and leave crops, the report says, and rural families lack remittances because migrant relatives are no longer travelling to Lebanon and Saudi Arabia to earn money.

ipj/ng (AP, dpa, AFP, Reuters)