Karzai welcomed the ruling by the government-appointed Independent Election Commission (IEC) after hours of closed-door meetings with western diplomats and mounting international pressure to accept the need for a second round.

The heavily disputed August 20 vote has fanned tension between Karzai and the West and complicated U.S. President Barack Obama's decision on whether to send thousands more U.S. troops to Afghanistan to fight a resurgent Taliban.

With U.S. Senator John Kerry by his side, Karzai told reporters at a news conference in Kabul that he accepted the IEC's ruling that a run-off must be held against his former foreign minister Abdullah Abdullah.

"We believe that this decision of the IEC is legitimate, legal and constitutional and that it strengthens the path toward democracy," Karzai said.

Afghanistan has endured weeks of political uncertainty that analysts say has only emboldened a resurgent Taliban after eight years of war.

The IEC's ruling came after a U.N. election fraud watchdog invalidated tens of thousands of votes for Karzai. That investigation, published on Monday, pushed the vote for Karzai back below the 50 percent level needed to avoid a run-off.

The decision however presents a logistical nightmare, with Afghanistan's harsh winter approaching fast.

MOUNTING CASUALTIES

The uncertainty has also added to pressure on Washington and Afghanistan's other allies, who have faced mounting casualties with violence at its worst levels since the Taliban were overthrown in 2001.

In an apparent show of solidarity, Karzai was also flanked by the United Nations' top representative in Kabul, Kai Eide, and the U.S. and British ambassadors.

Analysts say Karzai -- who is a Pashtun, Afghanistan's largest ethnic group -- is likely to win a run-off, but the scale of fraud alleged in the first round may cast a large shadow over the legitimacy of his rule.

Just before Karzai's announcement, a spokesman for the IEC confirmed that neither Karzai nor Abdullah had received more than 50 percent of the vote.

Provisional results had given Karzai 54.6 percent.

Kerry said Karzai showed "great leadership" in accepting the need for a second round, which "would allow the national leadership to run with legitimacy."

Kerry said that Abdullah had also agreed with the second-round verdict. He said there had been no discussion of a national unity government, suggested by some analysts as another way out of the impasse.

The West was ready to support the next round of the election even though it "required a sacrifice," Kerry said.

"A time of enormous uncertainty has been transformed into a time of great opportunity," Kerry said.

Earlier, U.S. Secretary of Defense Robert Gates said the United States cannot wait for problems surrounding the Afghan government's legitimacy to be resolved before making a decision on whether to send more troops.

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