The United States on Thursday blamed the resignation of UN peace envoy Kofi Annan on the refusal of Russia and China to back resolutions targeting Syrian President Bashar al-Assad.

Annan had "worked tirelessly to try to build consensus in the international community, end the bloodshed, and usher in a government that would meet the legitimate aspirations of the Syrian people," said Secretary of State Hillary Clinton.

"Unfortunately, the Security Council was blocked from giving him key tools to advance his efforts," Clinton said in a statement, adding she had thanked him for taking on the "heavy task" of trying to forge a peaceful transition.

White House spokesman Jay Carney, speaking aboard Air Force One, said Annan's departure also underscored the refusal of the Assad regime to halt murderous attacks against its own people.

"His resignation highlights the failure in the United Nations Security Council of Russia and China to support meaningful resolutions against Assad that would have held Assad accountable for his failure to abide by the Annan plan," Carney said.

"Those vetoes... were highly regrettable and placed both Russia and China on the wrong side of history and the wrong side of the Syrian people."

Carney also said that Obama appreciated the former UN secretary general's willingness to serve as envoy to Syria and for his efforts to bring about a peaceful transition amid fierce fighting between Assad's forces and rebels.

Annan said he quit as UN-Arab League envoy for Syria Thursday because of a lack of support and the increasing militarization of the conflict. "I did not receive all the support that the cause deserved," Annan said in Geneva.

He added that "continuous finger-pointing and name-calling" in the UN Security Council had hindered his attempts to implement the so-called six-point peace plan.

Russia and China have three times blocked possible sanctions against Syria, triggering outrage from the United States and its allies.

Clinton added that the United States still stood by the Syrian people and "we remain committed to an effective and swift political transition as envisioned under the Annan framework."

But Carney said Washington was sticking by its refusal to send arms to the Syrian opposition, despite rising calls to do so, and signs of an increasing US role in other areas.

"Our position has not changed: We provide non-lethal assistance to the opposition," Carney said. "We don't believe that adding to the number of weapons in Syria is what's needed to help bring about a peaceful transition."

On Wednesday, reports said Obama had signed a document authorizing covert US support to the rebels which authorizes clandestine action by the Central Intelligence Agency.

Earlier, the White House said that Obama had approved an extra $12 million in humanitarian aid for Syrians to mitigate what he said were Assad's "horrific atrocities."

The grant brings to $76 million the total amount of food, water, medical supplies, clothing, hygiene kits and other aid to some of the 1.5 million people in need in Syria.