UNITED NATIONS: UN Secretary General Ban Ki-moon warned Tuesday of "huge challenges" in holding the November 7 presidential election run-off in Afghanistan.

"There will be huge challenges to conduct it," Ban told reporters shortly after Afghan President Hamid Karzai agreed to hold a second round of voting following the country's fraud-marred August 20 contest.

Ban said he "warmly" welcomed the statement from Karzai, who "has made it clear that the constitutional process must be fully respected.

"This process is about the future of Afghanistan and the participation of the Afghan people in their future," he stressed.

The secretary general vowed that the United Nations would do its utmost to ensure the balloting is conducted "in a free, fair, transparent and secure environment." Karzai's decision came after an independent election commission determined he fell short of the 50 percent threshold needed to avoid a run-off against his main challenger, former foreign minister Abdullah Abdullah.

"I also commend Dr Abdullah Abdullah for his dignified approach to the campaign as well as to the difficult post-election period," said Ban.

The move was welcomed by Afghanistan's foreign allies as an opportunity to lift the country out of political chaos and help stabilize a nation blighted by an escalating Taliban insurgency.

After initial denials from the world body, a UN-backed watchdog on Monday threw out votes from 210 polling stations, raising pressure on Afghanistan's Independent Election Commission (IEC) to hold a run-off.

The UN-backed Electoral Complaints Commission (ECC) confirmed staggering levels of fraud in the August 20 vote, declaring more than one million suspect ballots -- a quarter of the total cast.

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