KABUL - Ballots from 10 percent of polling stations in Afghanistan's presidential election are to be recounted because of evidence of fraud, officials said Tuesday, exacerbating fears of protracted turmoil.

It will take weeks to complete the process, the officials said, plunging Afghanistan deeper into political limbo as Western troops battle to fend off a bloody Taliban insurgency.

Nearly a month after the polls, President Hamid Karzai leads an incomplete count with 54.3 percent of the vote. But his main rival Abdullah Abdullah -- trailing with 28.1 percent -- is calling for a run-off as fraud claims mount.

The UN-backed Electoral Complaints Commission (ECC) ordered that ballots from polling stations with "clear and convincing evidence of fraud" be recounted.

"About 2,500 plus polling stations are affected by the order and all provinces are affected," ECC chairman Grant Kippen told AFP.

The order said the Independent Election Commission (IEC) must identity polling stations with 100 percent turnout or where one presidential candidate received more than 95 percent of the vote.

Electoral authorities said more than 25,000 polling stations opened for the August 20 presidential and provincial council elections, seen as a key test of Western-backed efforts to rebuild the shattered country.

But the polls, Afghanistan's second presidential election and the first organised by Afghans, were marred by low turnout and widespread fraud claims, which have pushed back the scheduled announcement of the new president.

The drawn-out process has caused tensions to spike, as more than 100,000 US and NATO troops vie to contain a Taliban militia bent on destabilising the Western-backed Afghan government and local security forces.

The 2,500 polling stations will now be audited before the ECC decides whether to annul the ballots -- a time-consuming process.

"The idea is to do it as quickly as possible, so hopefully weeks as opposed to months is what we are looking at," said Kippen.

"Certainly the intent is to make this as thorough in the least amount of time as possible. It's difficult to say how long it will take."

Analysts and officials warned that most of the suspicious polling stations were in areas beset by the insurgency. An audit could therefore take "a long, long time," one IEC official said, on condition of anonymity.

"We're in the phase of political competition and political rivalry. Now we are entering the phase of political confrontation," said Haroun Mir, analyst from the Afghanistan Centre for Research and Policy Studies.

"We are heading for a political impasse," he said.

Mir said ballots from 10 percent of polling stations were unlikely to affect the final results given Karzai's huge lead, but warned that the number of stations under audit could rise.

A Western official close to the process warned, however, that if all votes from 2,500 stations were thrown out, Karzai could be hit hard.

"It can make a difference to the final outcome because most of the polling stations in question will be in the south, where there are legions of allegedly fanatical Karzai supporters," he told AFP.

The IEC has so far announced results from 95 percent of polling stations. An IEC spokeswoman said Tuesday there was still no timeframe for the final tally.

The official naming of Afghanistan's new president cannot take place until all vote-rigging complaints are investigated and resolved.

The IEC has already quarantined 600 polling stations for investigation, but it was not immediately clear if the 2,500 sites included those 600 polling stations.

The ECC has already thrown out votes from 83 polling stations -- most in Karzai strongholds -- based on complaints of fraud.