White House spokesman Jay Carney said Kofi Annan's resignation as mediator in Syria highlighted Syrian President Bashar Assad's refusal to abide by a UN-backed peace plan as well as the failure of Russia and China to hold Assad accountable at the UN Security Council.

"President Assad, despite his promise to abide by the Kofi Annan plan, continues to brutally murder his own people," he said.

The UN and Arab League had named Annan, a Nobel Peace Prize-winning former UN chief, as its envoy on the Syria conflict in February.

Assad agreed to a six-point peace plan with Annan but never carried it out.

German, British and French leaders were quick to express their frustration following Annan's resignation.

UN Secretary General Ban Ki Moon blamed "persistent division" within the UN Security Council as having become an "obstacle to diplomacy."

'"At a time when the Syrian people desperately need action, there continues to be finger pointing and name calling in the Security Council," Annan said. "As an envoy, I can't want peace more than the protagonists."

Russia and China, which had vetoed past UN Security Council draft resolutions on Syria, have both said they regretted Annan's decision.

On Friday morning there were opposition source reports of fresh clashes in the central city of Hama as well as Syrian army helicopter bombardment of the southern Hauran Plain, a strategic region that links Damascus with Jordan, where fighting has intensified in the last few days.

The UN general assembly will discuss the latest developments later on Friday in New York. It was due to vote on an Arab-drafted resolution.

The initial version called on Assad to stand down, but the latest draft has been toned down by removing the explicit call that Assad resign and that sanctions be supported.

rg/sej (AP, dpa, AFP, Reuters)