Kofi Annan says he is quitting his role as the U.N. and Arab League envoy to Syria, as violence from the country's 17-month-long uprising continues to spiral out of control.

Mr. Annan said Thursday that the increasing militarization on the ground and a lack of unity in the U.N. Security Council had changed the circumstances for him to effectively carry out his role.

In a news conference from Geneva, Mr. Annan said he would not renew his mission, which expires on August 31.

U.N. Secretary-General Ban Ki-moon expressed “deep regret” over Mr. Annan's departure. Mr. Ban blamed the Syrian government and the opposition for the continued unrest.

Both sides have failed to abide by a cease-fire which was part of a six-point peace plan brokered by Mr. Annan earlier this year. The truce began unraveling shortly after it took effect in April.

White House spokesman Jay Carney said China and Russia's stance in the U.N. has played a role in Mr. Annan's departure. Both countries have vetoed three U.N. Security Council resolutions on Syria that included action against the Syrian government.

Earlier Thursday, Russia said it would not support a draft resolution on Syria in the U.N. General Assembly. It said the non-binding measure that called for Syrian President Bashar al-Assad's departure was unbalanced.

The measure's Arab sponsors in the General Assembly later dropped language calling for Mr. Assad's resignation.

Carney also said there was no change in the U.S. position of providing non-lethal aid to Syria's opposition. He said adding to the number of weapons in the country would not help bring about a peaceful transition.

He commented on Thursday in response to Western media reports that President Barack Obama signed an order earlier this year allowing the Central Intelligence Agency and other U.S. agencies to provide support to the rebels.

In Syria, rebels have used a captured tank to shell a military base in the northern city of Aleppo, on Thursday, as government forces pound rebel strongholds in the city with tank and artillery fire.

The opposition Syrian Observatory for Human Rights says at least 60 people were killed in opposition-related violence across the country on Thursday.