Afghanistan will hold a second round of its presidential election on 7 November after incumbent Hamid Karzai failed to win a clear majority in the fraud-tainted contest, officials said Tuesday.

Exactly two months after polls that Karzai had been expected to win easily, the election commission confirmed he fell fractionally short of the 50 percent needed to avoid a run-off against main rival Abdullah Abdullah.

Led by the United States, Afghanistan's foreign allies welcomed the move as an opportunity to lift the country out of political chaos and help stabilise a nation blighted by an escalating Taliban insurgency.

"The election has gone to a second round. On 7 November it will be re-held," said Noor Mohammad Noor, spokesman for the commission.

Karzai confirmed at a news conference that he would take part in the second round, calling it a "step forward for democracy".

He spoke alongside UN envoy Kai Eide and US Senator John Kerry, whose presence underscored intensive Western lobbying of Karzai to resolve the weeks of political paralysis.

Karzai also urged the international community to help ensure the second round can pass off peacefully, with 100 000 US and NATO troops fighting record levels of Taliban violence, eight years after their regime was toppled.

"People need to cast their votes free of any security threats so that by the power of their ballots and votes they can build this country," he said.

US President Barack Obama welcomed Karzai's agreement to a second round as an important precedent for Afghan democracy.

'Huge challenges' - Ban

But UN chief Ban Ki-moon struck a note of caution, saying holding the run-off presented "huge challenges".

The announcement came a day after an inquiry by a UN-backed watchdog confirmed staggering levels of fraud in the 20 August vote, declaring more than one million ballots suspect ? a quarter of the total cast.

An election official confirmed that from a preliminary tally of 55 percent, Karzai's share of the first-round vote had fallen to 49.67 percent.

Karzai initially dismissed allegations of widespread fraud as fabricated, convinced he had a clear victory, but international pressure has been mounting.

"President Karzai has today made clear that due constitutional process must be followed," said British Prime Minister Gordon Brown, praising Karzai's "statesman-like" behaviour ? a phrase echoed by French leader Nicolas Sarkozy.

US Senator Kerry, standing shoulder-to-shoulder with Karzai, hailed the second election as a great opportunity for the country, where the United States has worked to put on a course to democracy since the Taliban overthrow.

Afghanistan had to hold a second round rapidly, before harsh winter sets in making much of the country inaccessible. But observers predicted weak turnout following a first round with a participation rate of only 38.7 percent.

The announcement seemed to nix suggestions that Karzai could join forces with Abdullah, his former foreign minister, in a government of national unity.

Abdullah welcomes run-off

Abdullah welcomed the second round, for which he has long agitated.

"Both Dr Abdullah and Hamid Karzai have insisted a second-round election take place," said his spokesperson Sayed Fazil Aqa Sancharki.

Abdullah's final percentage will be announced on Wednesday but a US-based monitor has predicted his share will rise from the 28 percent he received in preliminary results to nearly 32 percent.

There have been growing signs US patience with Karzai is wearing thin, as Obama wrestles with a decision on whether to deploy thousands more troops to Afghanistan.

Kerry, the powerful chairman of the Senate's foreign relations committee, has said it would be "entirely irresponsible" for Obama to commit more troops when the identity of the next Afghan government is still unclear.

A White House spokesperson said that Obama had yet to determine whether to make a decision on troop reinforcements before the run-off.