Afghan President Hamid Karzai has increased his lead over his top challenger, Abdullah Abdullah, and is nearing the 50 per cent mark needed to clinch re-election without forcing a runoff.

As of Saturday, Karzai has 46.2 per cent of the vote and Abdullah has 31.4 per cent, according to official tallies. However, results from only about a third of the country's polling stations have been counted so far.

Abdullah has intensified allegations that widespread fraud took place during the war-torn country's presidential poll, which took place on Aug. 20.

It was the second such election since U.S. troops ousted the Taliban regime there in late 2001, and the first election to be overseen by Afghan rather than international officials.

Since the vote, charges of fraud have flooded into the Electoral Complaint Commission, which is tasked with probing all such allegations before official results can be certified.

Fraud claims from presidential candidates, and lower voter turnout compared to the last presidential election in 2004, could hurt the Afghan government's legitimacy at home and internationally.

Holding credible elections is a key component of U.S. President Barack Obama's strategy for reconstructing the troubled state, in the face of a mounting Taliban insurgency.

NATO troops have been incurring more casualties in the war against the Taliban, leading to growing opposition to the mission in countries such as the U.S. and Britain. August has become the bloodiest month on record for U.S. troops there.

Voting results from northern Balkh province, where Abdullah was expected to run strong, are still far from complete. And few results have been reported from some southern provinces where Karzai was expected to draw significant support.

Members of Karzai's staff seemed convinced that the incumbent president would win a victory in the first round of voting, avoiding a runoff with Abdullah.

But Abdullah told Italy's RAI television that Karzai had perpetrated "state-crafted, massive election fraud."

"If we allow he who robbed the votes of this country to move forward, we would give the Afghan people a future that they do not want to see, and I think this goes also for the international community," Abdullah said.

"The foundations of this country have been damaged by this fraud, throwing it open to all kinds of consequences, including instability," he added. "It is true that the Taliban are the first threat to this country, but an illegitimate government would be the second."

Top officials from the U.S., the UN and the European Commission have applauded Afghans for voting in spite of threats of violence from the Taliban.

However, other international officials avoided calling the election credible as widespread fraud allegations surfaced in the days after the vote.

On Wednesday, officials from more than two dozen countries are due to meet in Paris to discuss Afghanistan. The election results will likely be a major point of dialogue at that meeting.

With files from The Associated Press