U.S. Secretary of State Hillary Clinton said Washington remains committed to an "effective and swift" political transition in Syria as envisioned by UN-Arab League joint envoy Kofi Annan, who announced his resignation on Thursday.

"We remain committed to an effective and swift political transition as envisioned under the Annan framework," Clinton said in a statement on Thursday.

She said she called Annan earlier in the day and thanked him for his service as the UN-Arab League joint envoy.

Clinton, currently on an African tour, attributed Annan's failure in his efforts to end the violence and bloodshed in Syria to a UN Security Council that "was blocked from giving him key tools to advance his efforts."

U.S. State Department spokesman Patrick Ventrell called the six-point peace plan brokered by Annan "a good framework."

Washington continues to support the framework within the context of a wider U.S. strategy of pursuing "robust" sanctions to squeeze the Syrian government while offering nonlethal assistance to the opposition as well as humanitarian aid to those affected by the conflict, he said.

"It's of course not surprising" that Annan chose to step down given the continued conflict in Syria and the lack of consensus at the Security Council, said the spokesman.

Annan, who assumed his current post about five months ago, told reporters in Geneva on Thursday that he was disappointed over the lack of progress in the Syrian peace process and that he would step down after his current mandate expires on Aug. 31.

The former UN chief said without united international pressure, it is impossible for him and anyone else to compel the Syrian government and the Syrian opposition to take the steps necessary to begin the political process.

"The increasing militarization on the ground and the clear lack of unity in the Security Council have fundamentally changed the circumstances for the effective exercise of my role," Annan said, adding he was frustrated at the "finger pointing" and "name calling" at the Security Council.

He criticized the Syrian government for its "continued refusal" of his peace plan and held the opposition responsible for the escalation of the military campaign which has centered on the capital city of Damascus and the northern province of Aleppo for the past days.

China expressed regret and understanding over Annan's decision to resign, Chinese Foreign Ministry spokesman Hong Lei said Thursday.

China is committed to the peaceful, fair and appropriate resolution of the Syrian issue, and the international community should stick to the political settlement of the issue, said Hong.

As permanent members of the Security Council, Russia and China have advocated a negotiated settlement of the conflict by concerned parties in Syria, and opposed any foreign interference.