"It is important that the outcome of these elections reflect the will of the Afghan people.

"We will continue to encourage Afghan authorities to follow the comprehensive anti-fraud measures established in order to protect the integrity of the election process and ensure that the election results are credible."

There are growing fears that Afghanistan's second presidential election is at risk of being undermined by a low turn-out and widespread allegations of vote-rigging.

The condemnation follows reports that President Hamid Karzai had heated exchanges with Richard Holbrooke, President Obama's special envoy to the region, during a discussion about the August 20 poll.

Envoys from Britain, the US and its allies are also to meet on Wednesday for a summit in Paris to discuss the aftermath of the vote.

The Afghan election watchdog has received more than 1,500 accusations of ballot rigging, intimidation and fraud.

Abdullah Abdullah, the former foreign minister and the president's closest challenger, has accused the government of "state-engineered fraud" and said he will not accept the legitimacy of a stolen election.

Yesterday the incumbent Hamid Karzai appeared to pull farther ahead of his main rival, according to the latest results unveiled yesterday by the country's election commission.

Total results released so far come from 35 percent of polling stations

Out of 2.03 million valid votes counted, Karzai won 940,558 and former foreign minister Abdullah Abdullah 638,924, Daud Najafi, chief electoral officer at the Independent Election Commission (IEC), told a news conference.

Those results handed Karzai 46.3 percent of votes announced and Abdullah 31.4 percent, widening the incumbent's previous lead of around nine percent.

Dr Abdullah's campaign has filed more than 100 official complaints against the Karzai campaign including allegations that ghost polling stations were set up and ballot boxes were stuffed with forged votes.

Mr Karzai's campaign has in turn accused Dr Abdullah's supporters of intimidation.

The incumbent is widely considered to be the favourite to win, but pre-election polls suggested Dr Abdullah could deny him the 50 per cent needed to avoid a second round.

After polling, Mr Karzai's campaign has repeatedly said it had gained enough votes to win outright.

A successful Taliban campaign of intimidation meant most voters were scared away in much of the Pashtun tribal south and nationwide turnout may be as low as 35 per cent.

Certified results will not be announced in the election until September 17.