The United States says Kofi Annan's resignation as UN envoy to Syria highlights the failure by Russia and China to back meaningful resolutions in the Security Council.

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.

Mr. Annan said Thursday he is quitting as envoy, saying he cannot be the only one working for peace while the fighting between Syrian forces and the rebels gets worse. He also blamed what he calls a lack of unity in the Security Council.

U.N. Secretary-General Ban Ki-moon expressed deep regret over Mr. Annan's departure.

Both sides have 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 Syrian President Bashar al-Assad responsible for his failure to abide by the Annan plan and threatened him with sanctions.

Caarney says the summary executions of suspected pro-government militiamen by Syrian rebels is one reason the United States will not supply weapons to the opposition. He says adding more weapons will not help bring about peace.

In Syria, rebels used a captured tank to shell a military base in the northern city of AleppoThursday, 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.

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