FILE - In this Monday, Oct. 26, 2009 picture, Abdullah Abdullah, former Afghan foreign minister who ran against President Hamid Karzai in August's vote, gives a news conference in Kabul, Afghanistan. [Agencies]

President Hamid Karzai's challenger plans to call for a boycott of next weekend's runoff election in an attempt to force the vote's postponement until spring, his campaign manager said — a move that would dim US hopes for a stable Afghan government for months.

Karzai rejected Abdullah Abdullah's conditions for next Saturday's vote, including removing top election officials whom the challenger accused of involvement in cheating in the first-round balloting in August.

Abdullah has called a press conference for 10 am Sunday to announce his final decision after Afghans and Westerners close to the challenger said he would withdraw. His campaign manager Satar Murad said the candidate might still change his mind, but that "as of now" he planned to call for a boycott.

A clouded electoral picture would further complicate the Obama administration's efforts to decide whether to send tens of thousands more troops to Afghanistan to battle the Taliban and its al-Qaida allies.

The White House has been waiting for a new government in Kabul to announce a decision, but the war has intensified in the meantime. October was the deadliest month of the war for US forces with at least 57 American deaths.

Western officials hoped that Abdullah would make a gracious exit for the good of the country rather than denounce Karzai for fraud, a move that could sharpen tensions at a time the United States and its allies are seeking unity against the Taliban.

"I don't think it has anything to do with the legitimacy of the election," Clinton told reporters in Abu Dhabi. "It's a personal choice which may or may not be made."

Last-minute contacts were under way late Saturday between representatives of Karzai and Abdullah to resolve the crisis, and the challenger's spokesman Fazel Sancharaki insisted no final decision on a withdrawal had been made. It appeared the uncertainty was aimed in part at allowing Abdullah to keep his options open until the last possible moment.

"I don't know what will happen tomorrow morning," Murad said. "We understand it shouldn't go forward, and there should be an interim government immediately after the 7th or 8th of November" until a vote next spring, Murad said.

He said the approach of winter meant there was not enough time to organize an election that would be reasonably free of fraud.

"Therefore, it's not good for the country, and it's not good for the people," Murad said. "We wouldn't be having a legitimate government in the country if we went forward."

If the election proceeds as scheduled, Abdullah will urge supporters to stay at home, and "our followers will not turn up to the election centers."

US officials pressured Karzai into agreeing to a runoff after UN-backed auditors threw out nearly a third of his votes from the August ballot, citing fraud. Obama administration officials said they would be receptive to a power-sharing deal to avoid a runoff if Karzai and Abdullah could agree on a formula.

But Abdullah decided to exit the race after talks between the two sides broke down Thursday, according to two people close to the negotiations. They spoke on condition of anonymity because they were not authorized to speak for the candidate.

During recent talks, Abdullah demanded the removal of three key election officials, suspension of three Cabinet members and constitutional changes that would give him a say in the appointment of ministers and in major policy decisions, according to an Afghan close to the Karzai campaign.

Karzai refused the to agree to the conditions, the Afghan said, speaking on condition of anonymity because he was not supposed to talk about the confidential discussions.

Even if Abdullah withdraws, it's unclear whether Karzai could be proclaimed the winner or if the runoff would still have to proceed, either with Abdullah on the ballot or the third-place finisher, lawmaker Ramazan Bashardost.

A spokesman for the Afghan election commission said that it is too late for Abdullah to officially withdraw and that a boycott will not prevent the runoff from going forward.

"The election will be held and all procedures will go as normal," Noor Mohammad Noor said.

US officials have been concerned that the second round would expose Afghan civilians to attack by Taliban militants opposed to the election.

Last Wednesday, Taliban attackers killed five UN employees — including one American — and three Afghans in a brazen assault on a residential hotel housing international staff in the heart of Kabul. The three attackers also died.

Afghan intelligence chief Amrullah Saleh said Afghan authorities had advance information that a Taliban attack in Kabul was in the works but was expected it during rush hour, and officials were unsure of the target.

Instead, the attackers struck just before dawn. Saleh said eight people had been arrested for their roles in the attack, including an Afghan imam who was apprehended when he arrived by plane in Jiddah, Saudi Arabia.

Saleh said the detainees told interrogators the attackers came from Pakistan's Swat Valley and that the al-Qaida mastermind fled across the border into Pakistan's lawless tribal area, where the al-Qaida leadership is believed hiding.

Casualties have been on the rise since President Barack Obama sent more troops to confront the Taliban.

On Saturday, the NATO-led force announced the latest coalition death in the war. The Canadian Defense Department said the casualty was a 24-year-old Canadian national killed in a bomb blast outside the southern city of Kandahar on Friday.