WASHINGTON: US 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," Hillary 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.

Hillary, 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”.

US 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 US 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 August 31.