"The UN, NATO, the US stand ready to assist the Afghans in conducting the second round," White House spokesman Robert Gibbs told reporters.

"Whether or not the president makes a decision before that I don't think has been determined.

"I have continued to say a decision will be made in the coming weeks as the president goes through an examination of our policy," he added.

Gibbs also praised Secretary of State Hillary Clinton, US ambassador to Kabul Karl Eikenberry and especially Senator John Kerry, who has been mediating in Afghanistan for several days, for their role in ending the election limbo.

"I don't think there is any doubt that Senator Kerry played an enormously important role in ensuring the announcement that happened today... came out the way it did."

Afghan officials announced earlier that they will hold a runoff presidential election, after incumbent President Hamid Karzai failed to win a clear majority in the fraud-tainted contest.

Obama has been conducting an intensive review of US war strategy in Afghanistan with senior national security officials.

Part of his decision centers on US and NATO war commander General Stanley McChrystal's request for some 40,000 more US troops to mount an intensified counter-insurgency against the Taliban in Afghanistan.

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