KABUL - An Afghan election watchdog ordered ballots from more than 200 polling stations to be invalidated Monday after an inquiry concluded there had been large-scale fraud in August's presidential poll.

The Electoral Complaints Commission (ECC) said it had found "clear and convincing evidence" of fraud in the August 20 poll, including entire ballot boxes which had papers filled in with the same pen or with the same mark.

It said it had ordered the UN-backed Independent Election Commission (IEC) -- the final arbiter of the election results -- "to invalidate a certain percentage of each candidates' votes" and results from 210 polling stations.

The ECC findings, which it said were "final and binding," could see the IEC forced to call a run-off between incumbent President Hamid Karzai and his main rival Abdullah Abdullah.

Karzai, against whom most of the ballot-stuffing allegations were made, leads preliminary results with about 55 percent of the vote, with Abdullah on about 28 percent.

But the revised count is expected to cut Karzai's lead to below the mark of 50 percent plus one vote required for victory.

One Western diplomat said Karzai's support had fallen to around 48 percent, but the ECC refused to confirm the claim.

But amid fears that the ECC findings may not be accepted -- and concerns about a political vacuum -- the United Nations' spokesman in Kabul Aleem Siddique called for the watchdog's orders to be implemented soon.

"Now that the ECC has published its order we expect the IEC to implement with haste and move swiftly to announce either a final certified result or the requirement for a second round as required by Afghan electoral law," he said.

In a statement, the US embassy said: "We call on the IEC to implement these orders with all due speed and look forward to the final certified results."

The ECC announcement comes as patience among Afghanistan's international backers wears thin, with the political crisis poised to head into a third month and Karzai under huge pressure to accept the results.

Abdullah supporters have said that they will not accept a Karzai victory because of the vote-rigging claims, raising fears about the next administration's legitimacy at a time of spiralling insurgent violence.

White House chief of staff Rahm Emanuel and Senator John Kerry both said at the weekend that it would be "irresponsible" to send more troops to Afghanistan to fight the resurgent Taliban before the election is resolved.

"I think it would be irresponsible and... reckless to make a decision on US troop level if, in fact, you haven't done a thorough analysis of whether in fact there's an Afghan partner ready to fill that space," Emanuel told CNN.

US President Barack Obama is considering a request from his military commanders for another 40,000 troops to boost the more than 100,000 already in Afghanistan under US and NATO command.

Emanuel's words echoed those of Kerry, chairman of the Senate foreign relations committee and a close Obama ally, who met Karzai and Abdullah in Kabul at the weekend.

Karzai's survival -- and that of his government -- depends on the continued support of the international community, which has poured billions of dollars in aid into the country since overthrowing the Taliban regime in 2001.