Updated

The Taliban's brazen attack on U.N. election workers undermines the U.N.'s ability to help steer Afghanistan through a runoff election in only 10 days. Although the U.N. insists it will not be deterred by the assault, another big attack could derail its limited ability to assure a credible vote and remain in the country.

Most of the U.N.'s international staff in Afghanistan were ordered to stay home Thursday, a day after militants stormed a residential hotel housing U.N. employees, killing five of them, including one American. Six other people died, including the three attackers.

The lockdown does not apply to the 140 U.N. personnel helping the Afghans prepare for the Nov. 7 presidential runoff, according to U.N. spokesman Dan McNorton. Time is running out to arrange a ballot that already faces threats ranging from Taliban violence to possibly early winter snow.

"They're going to warehouses or meetings, or the airport to check on logistics," McNorton said of U.N. election workers.

He said technical advisers will be sent to the provinces in time for the vote, although all assignments will be reviewed to make sure the staff is reasonably safe.

"Yesterday was obviously a disruption, but the work and the support that we're providing remains strong and is working," he said.

In New York, U.N. Secretary-General Ban Ki-moon appealed for more security personnel to protect U.N. staff and facilities in Afghanistan, especially in the run-up to next week's election.

Ban said he urged members of the 15-nation Security Council on Thursday to provide additional security units and would make a similar appeal Friday to the 192-member General Assembly.

Wednesday's attack was only the latest in a series of setbacks suffered by the U.N. mission assisting the Afghans in running an election on their own for the first time since the U.S.-led invasion brought down the Taliban government in late 2001.

The top-ranking American in the U.N. Assistance Mission in Afghanistan was fired after a public dispute with the mission's Norwegian chief over whether he had been bullish enough in preventing fraud in the Aug. 20 first-round vote.

The fraud allegations seriously undermined the credibility of President Hamid Karzai's election, forcing him into the runoff. Still, the Afghan election commission brushed aside a U.N. recommendation to reduce the number of polling stations to curb cheating and decided Thursday to open even more in the November ballot.

The Taliban regards the election as a Western plot. A credible result would do much to undermine its claim to be the only valid form of government for the nation of some 30 million.

The U.N. presence, which includes several hundred foreign staff, is essential to ensure the election meets an acceptable standard of fairness. Without that, the government's legitimacy as a credible partner with the U.S. and its allies in the fight against the Taliban would be in doubt.

U.N. officials were quick to quell speculation that the threats might drive out the world body.

"Just because somewhere is difficult and dangerous doesn't necessarily mean we will not be able to be there," McNorton said.

Nevertheless, Wednesday's daring daylight attack in the heart of Kabul has sent shock waves through the city's international community, which has generally enjoyed a freer life than their counterparts in Baghdad.

In addition to the guest house attack, Taliban militants fired rockets at the Serena, the country's most luxurious hotel. No one was hurt but dozens of Westerners and well-heeled Afghans fled into the basement when the lobby filled with smoke.

"Our work continues, and in terms of the elections, preparations are already well advanced," said Aleem Siddique, another U.N. spokesman. "But the impact this will have needs to be evaluated over the coming days, and it's too early to make any judgments."

Some Westerners said they are weighing whether Afghanistan has become too risky. They would not allow their names to be used for security reasons — and to avoid alarming their families back home.

The Taliban have in the past staged attacks against Western civilians, among the most dramatic the January 2008 assault against the Serena's gym. Six people, including a Norwegian journalist and two attackers, were killed.

Still, most attacks had been followed by long periods of calm in the capital, reinforcing a sense that Kabul was removed from the violence that grips other parts of the country.

This time may be different.

Wednesday's assault was the fourth major attack in Kabul since a suicide car bomber struck Aug. 15 near the front gate at NATO headquarters, killing at least seven people and wounding about 100. That blast was followed by a suicide attack Sept. 17 that killed six Italian soldiers and 10 Afghan civilians, and a suicide car bombing Oct. 8 at the Indian Embassy in which 17 died.

Another major attack ahead of the election — with large loss of life among U.N. and other international personnel — could well change the security equation. The August 2003 truck bombing at the U.N. headquarters in Baghdad, which killed 22 people, including mission chief Sergio Vieira de Mello, prompted the U.N. to shut down operations in Iraq for years.

So far, the U.N. has not ordered a general evacuation of its international staff, but those not working on the election have been encouraged to take vacations or work outside the country until the runoff is over.

An internal U.N. memo ordered restrictions on movement for the rest of the week and said U.N. departments would review lists of critical and nonessential personnel, suggesting some people may be moved to safety outside the country.

Threats against aid groups are also on the rise.

The Agency Coordinating Body for Afghan Relief, an umbrella group of more than 100 local and international agencies, said attacks on its member organizations are at their highest in six years, with at least 23 workers killed this year, and many groups have had to restrict operations.

One agency, the U.N. women's fund, evacuated most of its international staff before the first round of voting on Aug. 20, brought them back and then sent them out again following Wednesday's attack, McNorton said.

The Geneva-based U.N. High Commissioner for Refugees said it was assessing the impact of Wednesday's attack on its work. The U.N. Food and Agriculture Organization will remain in Afghanistan "as long as the security situation would permit," spokesman Erwin Northoff said in Rome.