Hamid Karzai and Abdullah Abdullah have ruled out a power-sharing deal
Afghan President Hamid Karzai has rejected a call by rival presidential candidate Abdullah Abdullah to sack the head of the Afghan election commission.
Mr Abdullah made his demand ahead of a 7 November run-off, after a UN-backed panel threw out first-round votes.
He said commission chief Azizullah Lodin had "no credibility", but Mr Karzai said he had done nothing wrong.
The row came as US President Barack Obama pledged not to "rush" a decision about whether to send extra US troops.
Mr Obama held a sixth meeting with his national security team on Monday to discuss the future US strategy in Afghanistan.
He then spoke at a military base in Florida, telling troops he would "never hesitate" to use force if necessary.
But, he added: "I will never rush the solemn decision of sending you into harm's way."
In Afghanistan the incumbent president dismissed Mr Abdullah's allegations, saying that sudden upheaval could upset the voting process.
"The changes would not be helpful to the elections and the country," Hamid Karzai said.
A UN-backed panel found evidence of widespread fraud in the first round
Mr Abdullah and his aides insist the IEC is dominated by officials loyal to Mr Karzai, and the challenger has called for some to be removed from their posts.
However, Mr Karzai, who appointed the commissioners, said they "have just done their legal duties".
There has been no statement from the IEC or Mr Lodin as yet.
Earlier, Mr Abdullah and Hamid Karzai earlier ruled out a power-sharing deal.
Both candidates told US media they were committed to another poll.
Speaking to CNN, Mr Karzai - who bowed to international pressure to hold a run-off - said a deal would be "an insult to democracy".
List of conditions
Mr Abdullah made his demand for Mr Lodin's dismissal during a news conference at which he outlined a list of conditions for a fair second round.
KARZAI V ABDULLAH
First popularly elected president of Afghanistan
Opposed Soviet occupation in 1980s
Critics say he has done little to rein in corruption
Tajik-Pashtun, doctor by profession
Senior Northern Alliance leader during Taliban rule
Removed from Karzai's cabinet in 2006
"He has left no credibility for the institution," Mr Abdullah was quoted by the AFP news agency as saying.
"What's the solution? Another commissioner from the same commission should take his position," he said.
After the 20 August poll, initial results suggested that Mr Karzai had received 55% of the vote, and former foreign minister Mr Abdullah 28%.
But the UN-backed Electoral Complaints Commission (ECC) deducted hundreds of thousands of votes from the main candidates.
Its investigation focused on 600 of the most serious complaints, and a sample audit of suspect votes at 3,377 polling stations. At 210 polling stations all the ballots were invalidated.
This meant Mr Karzai's total was reduced to below the 50% plus one vote threshold for outright victory, indicating a second round was needed.
The panel also recommended replacing thousands of officials and scrapping polling stations where the fraud was worst.
Officials involved in flawed polling are being removed ahead of the run-off, the UN has said.
But there are still concerns about the ability of the run-off to avoid mistakes made in the first round, correspondents say.
Campaigning officially began over the weekend but the Taliban threatened to launch a fresh wave of violence and urged people not to vote in what they called an "American process".