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KABUL (Reuters) - President Hamid Karzai agreed to face a second round of voting in Afghanistan's disputed election on Tuesday after a U.N.-led fraud inquiry tossed out enough of his votes to trigger a run-off.

Election workers audit and recount ballots, at a warehouse in Kabul October 5, 2009. President Hamid Karzai agreed to face a second round of voting in Afghanistan's disputed election on Tuesday after a U.N.-led fraud inquiry tossed out enough of his votes to trigger a run-off. (REUTERS/Ahmad Masood/Files)

The dispute around the Aug. 20 vote has stoked 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.

Obama, British Prime Minister Gordon Brown and U.N. Secretary-General Ban Ki-moon all welcomed Karzai's decision to accept the run-off.

"It is now vital that all elements of Afghan society continue to come together to advance democracy, peace and justice," Obama said in a statement.

"We look forward to a second round of voting, and the completion of the process to choose the president of Afghanistan."

The White House said the president had taken no decision on whether to wait for the new poll before announcing a new strategy for Afghanistan. A decision would be taken "in the coming weeks".

The Nov. 7 election will pit Karzai once again against his main election rival, former foreign minister Abdullah Abdullah.

After hours of closed-door talks with Western diplomats, Karzai appeared tense as he welcomed the ruling by the Afghan Independent Election Commission (IEC). The ruling cut his tally to 49.7 percent from the preliminary first-round result of 54.6 percent -- below the 50 percent needed for an outright win.

"We believe that this decision of the IEC is legitimate, legal and constitutional and that it strengthens the path towards democracy," said Karzai, U.S. Senator John Kerry by his side and U.N. Afghanistan envoy Kai Eide standing between them.

The IEC made its ruling after a separate U.N.-backed fraud panel invalidated tens of thousands of votes for Karzai this week. Karzai had earlier said the extent of fraud was exaggerated and expressed confidence in his first-round victory.

Abdullah's camp said they were prepared for the run-off.

"We had hoped the president would accept the second round," said his spokesman, Fazel Sangcharaki.

MOUNTING CASUALTIES

Karzai, who is a Pashtun, Afghanistan's largest ethnic group, is almost certain to win the run-off but the level of mass fraud alleged in the first round will inevitably cast a shadow over the new vote.

Security issues are also of concern at a time when the insurgency is at its strongest and winter approaches.

"The Taliban no doubt will try their best to disrupt it," said Waheed Mozhdah, an Afghan analyst. "It (run-off) will be difficult if our intention is for a better and transparent election compared to the first round."

Kerry said holding the second round would be tough in the present environment. But the West, he said, was committed to assisting Afghanistan.

"We know it will be difficult and require sacrifice," he said. "But we are committed to this effort."

The uncertainty, however, has added to pressure on Washington and Afghanistan's other allies, Britain in particular, which face mounting casualties as violence this year reached its worst levels since the Taliban were overthrown in 2001.

In Washington, White House spokesman Robert Gibbs said it was not certain Obama would announce a new strategy before the run-off.

"Whether or not the president makes a decision before that I don't think has been determined," he told reporters. "I continue to say that the decision will be made in the coming weeks as the president goes through an examination of our policy."

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

(Additional reporting by Golnar Motevalli, Maria Golovnina and Sayed Salahuddin in KABUL, Ross Colvin in Washington, and Louis Charbonneau at the United Nations; Writing by Golnar Motevalli and Maria Golovnina)