Kabul  - UN Secretary General Ban Ki-moon arrived Monday on an unannounced visit to Kabul as political uncertainty deepened in Afghanistan after the pullout of presidential challenger Abdullah Abdullah from a planned runoff election.

Ban came to the war-torn country six days after Taliban militants, equipped with suicide vests and automatic rifles stormed a UN guesthouse in downtown Kabul, killing five UN international staff and three Afghans.

UN spokesman Aleem Siddique said the secretary general was in Kabul to show his "solidarity" with UN staff members in Afghanistan after the deaths Wednesday.

Ban was to meet with President Hamid Karzai for lunch, then with Abdullah in the afternoon and later with UN employees, Siddique said.

The Taliban claimed the attack in Kabul was the beginning of the their anti-election campaign because the UN staff who were attacked were helping convene the second round of the elections after the August 20 presidential vote was marred by massive fraud, mostly in favour of Karzai.

Ban was expected to discuss the election situation with Karzai and Abdullah after Abdullah on Sunday pulled out of Saturday's runoff. The former foreign minister said he withdrew after Karzai did not accept his conditions for bringing transparency to the elections.

Abdullah's decision to withdraw left Karzai as the only candidate and brought him to the verge of remaining in office, but it also cast doubt over the legitimacy of his future administration.

Both Karzai's camp and Independent Election Commission said the runoff would go ahead despite Abdullah's decision, but Western officials said the international community, which provided funds and security for the election, was not willing to support a one-man election.

"There is no appetite on the part of international community for the second round of the elections," said a Western official who declined to be named because of the sensitivity of the issue.

He said it would be "ridiculous" to spend money and risk lives for a process in which the outcome is already known.

The Taliban have vowed to disrupt the fresh vote.

Karzai garnered more than 54 per cent of the ballots in the August election, a percentage that made him an outright winner, but a UN-backed investigation discounted about 1 million, or a third, of his ballots and pushed him into a runoff with Abdullah, his nearest rival.

The Afghan constitution failed to anticipate a situation in which a candidate pulls out of a runoff, and officials said they believe Afghanistan's Independent Election Commission or the Supreme Court has to come up with a ruling to legitimize the future government.

A commission spokesman said the agency's six-member leadership was meeting Monday to reach a consensus on whether to go ahead with the runoff or find another legal remedy to end the election crisis. (dpa)