Kofi Annan says he has received no support from world powers to take decisive action over the escalating violence in Syria, and has decided to resign as the United Nations special envoy to Syria as of Aug. 31.

UN chief Ban Ki-moon announced the surprise resignation Thursday.

"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 August 31, 2012," Ban said in a statement.

“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.”

At an impromptu press conference in Geneva, Annan said that when he accepted the role of peace envoy in February, it seemed the international community could help end the months of violence, and bring about a political transition.

But Annan told reporters he cannot go on when the 15-nation Security Council provides no backing for his role and continues to bicker amongst itself.

"When the Syrian people desperately need action, there continues to be finger-pointing and name calling in the Security Council," Annan told reporters.

"It is impossible for me or anyone to compel the Syrian government and also the opposition to take the steps to bring about the political process," he said.

 "You have to understand: As an envoy, I can't want peace more than the protagonists, more than the Security Council or the international community for that matter."

Four months ago, Annan helped draft a six-point peace plan that was intended to bring an end to the uprising that has resulted in the deaths of more than 19,000 Syrians. Yet, despite the presence of hundreds of UN observers on the ground, neither side in the conflict has fully adhered to the plan and the violence has continued.

Annan did not rule out the idea of a successor being appointed, since "the world is full of crazy people like me, so don't be surprised if someone else decides to take it on."

The resignation comes a day before the UN General Assembly is due to vote on a non-binding resolution drafted by Arab League countries.

The resolution was originally going to call on Syrian President Bashar Assad to resign and turn over power to a transitional government. But on Thursday, it was decided to drop the demand for Assad’s resignation.

It appears the so-called BRIC nations, including Brazil, Russia, India, China and South Africa, objected to calling for a regime change.

CTV’s Middle East Bureau Chief Martin Seemungal told CTV News Channel that he didn’t think the timing of Annan’s resignation, one day before the UN vote, was a coincidence.

Seemungal said the UN vote is likely to pass, but is unlikely to help quell violence in Syria. The vote doesn’t bring any sanctions.

“It won’t change anything on the ground and I think this is what Annan realizes. Unless people on the ground are listening to calls for a ceasefire, nothing will happen,” he said.

Ban said in a statement he accepted the resignation with deep regret, and is discussing possible successors with the Arab League.

"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.

Ban said the UN remained committed to “pursue through diplomacy” an end to the violence.

“This can only succeed – indeed any peacemaking effort can only prosper – when the parties to the violence make a firm commitment to dialogue, and when the international community is strongly united in support.”

Violence escalates

Meanwhile, the battle for Syria intensified Thursday as rebels pounded a military base in the historic city of Aleppo.

The rebels captured a tank from Syrian soldiers and used it in combat. Observers say that kind of co-ordination could translate into an even bloodier civil war.

There was also heavy shelling around the rebel-held town of Azaz on the Turkish border Thursday.

In the capital, Damascus, the Syrian regime said it arrested and killed a number of "terrorists" -- the government's term for its opponents -- in a string of raids.

With files from The Associated Press