Abdullah Abdullah gestures after announcing his decision not to participate in Afghanistan's runoff election during a news conference Sunday in Kabul. (Gemunu Amarasinghe/Associated Press)

President Hamid Karzai's challenger has withdrawn from next weekend's run-off election — effectively handing the incumbent a victory — but Abdullah Abdullah stopped short of calling on his supporters to boycott the vote.

The country's former foreign minister announced his decision on Sunday, saying Karzai turned down his proposals to prevent fraud in the run-off, scheduled for Nov. 7 in the wake of widespread claims of fraud during initial balloting in August.

"As far as I'm concerned, the decision I have reached is not to participate," Abdullah later told reporters. "I have strong, strong reservations about the credibility of the process."

Abdullah had complained that Karzai turned down his demands for changes in the Independent Election Commission and other measures that Abdullah said would prevent massive fraud in the second round of balloting.

Abdullah stopped short of calling for an electoral boycott and urged his followers "not to go to the streets, not to demonstrate."

Azizullah Lodin, the head of the Karzai-appointed commission, said he would have to confer with constitutional lawyers before deciding later Sunday whether the run-off would proceed without Abdullah.

Abdullah's running mate, Homayoun Assefy, said it was up to the government's Independent Election Commission to decide whether to hold the run-off next Saturday as scheduled.

Obama weeks from troops decision

The clouded Afghan electoral picture further complicates the U.S. administration's efforts to decide whether to send tens of thousands more troops to battle the Taliban and its al-Qaeda allies.

David Axlerod, a senior adviser to Barack Obama, said the U.S. president is still weeks away from deciding whether to send more troops. Axlerod said most polls showed Abdullah would have lost the run-off anyway.

"So we are going to deal with the government that is there," Axlerod said on CBS News' Face the Nation. "And obviously there are issues we need to discuss, such as reducing the high level of corruption. These are issues we'll take up with President Karzai."

Secretary of State Hillary Rodham Clinton, travelling in Morocco, said the U.S. will support the next president and the people of Afghanistan, "who seek and deserve a better future."

She said in a statement that Abdullah ran "a dignified and constructive campaign" that won the backing of many in his country. "It is now a matter for the Afghan authorities to decide on a way ahead that brings this electoral process to a conclusion in line with the Afghan constitution," according to the statement released by the State Department.

U.S. officials had pressured Karzai into agreeing to a run-off after UN-backed auditors threw out nearly a third of his votes from the August ballot, citing fraud.

Vote should proceed, Karzai spokesman says

Karzai's campaign spokesman, Waheed Omar, said it was "very unfortunate" that Abdullah had withdrawn but that the Saturday run-off should proceed.

"We believe that the elections have to go on, the process has to complete itself, the people of Afghanistan have to be given the right to vote," Omar said.

Canada's ambassador to Afghanistan called on both candidates to respect the electoral process.

"We have consistently stated that all parties must respect the process to enable Afghans to exercise their democratic rights," William Crosbie said in a statement Sunday to The Canadian Press.

"We will monitor the situation closely for any developments, and we await the response from the Independent Election Commission on the next steps in the Afghan electoral process," he said.

With files from The Associated Press and The Canadian Press