United Nations peacekeeping chief Herve Ladsous says the main battle for the Syrian city of Aleppo is about to begin.

Ladsous told reporters after a Security Council meeting Thursday that observers on the ground say the violence there is getting worse.

Syrian forces have been pounding the country's largest city with tanks and helicopters for nearly three weeks trying to seize control from the rebels who want to topple President Bashar al-Assad. Hundreds of thousands of civilians have fled and an unknown number have been killed.

United Nations envoy to Syria, Kofi Annan, announced Thursday that he is quitting. He said he cannot be the only one working for peace while the fighting gets worse. He also blames what he calls a lack of unity in the Security Council.

The United States says Mr. Annan's resignation highlights the failure by Russia and China to back meaningful resolutions.

White House spokesman Jay Carney says the Russian and Chinese vetoes of three anti-Assad drafts are highly regrettable and put both nations on the wrong side of history.

Both the Syrian rebels and the government failed to commit themselves to Mr. Annan's peace plan for Syria, which included an immediate cease-fire and talks on a transitional government. The three Security Council resolutions that China and Russia vetoed would have held President Assad responsible for his failure to abide by the Annan plan and threatened him with sanctions.

The Council Thursday also debated whether to renew the mandate of the U.N. observer mission for Syria. The mission expires on August 19th. French Ambassador and current Council president Gerard Araud expressed doubt whether the mission would be extended. Russian Ambassador Vitaly Churkin says he strongly backs a renewal.