Traumatized U.N. staff in Afghanistan were under orders to stay home Thursday, one day after Taliban militants stormed a guest house in the capital and killed eight people in a brazen attack that is forcing the world body to re-evaluate its mission in the war-ravaged nation.

The attack underscored the risks facing U.N. and Afghan officials in organizing a runoff election following the fraud-marred first-round vote Aug. 20, and the massive challenge for the U.S.-led military force in curbing the determined Taliban insurgency. NATO said two members of its military operation in Afghanistan were killed in bomb blasts in the south Wednesday, including one American.

The guest house assault, which left five foreign U.N. staff and three Afghans dead, demonstrated the ease with which Taliban militants can penetrate the relative safety of Kabul. A Taliban spokesman said the attack was aimed at undermining the Nov. 7 presidential election runoff; the target was a small hotel home to the largest concentration of U.N. staffers working on the election.

SLIDESHOW: Deadly Taliban Attack in Kabul

U.N. spokesman Aleem Siddique said at least nine U.N. staff who survived the two-hour assault on the Bakhtar guest house will be evacuated to Dubai in the United Arab Emirates on Thursday.

The U.N. has ordered its employees to remain "on lockdown," with their movements restricted, Siddique said Thursday, declining to give details for security reasons. Another U.N. staffer said that meant most staff were staying home.

An internal U.N. memo ordered restrictions on movement for the rest of the week and said U.N. departments will be reviewing lists of critical and nonessential personnel, suggesting some people may be moved out of the country for their own safety.

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

The Aug. 19, 2003, truck bombing of the U.N. headquarters in Baghdad, which killed 22 people, prompted the U.N. to pull out of Iraq for several years.

An umbrella group of more than 100 local and international aid agencies said aid organizations in Afghanistan "have been subject to increasing numbers of attacks, threats and intimidation by both insurgent and criminal groups."

The Agency Coordinating Body for Afghan Relief said attacks on aid groups are "higher now than in the last six years." Wednesday's violence brought to the number of aid workers killed in Afghanistan this year to at least 23, the group said. Fifteen more have been injured since the start of the year.

"This situation has forced many aid agencies to restrict the scale and scope of their development and humanitarian operations," the group said. "Deteriorating security conditions continue to jeopardize the delivery of essential humanitarian assistance and threaten the livelihoods of the most vulnerable."

The five U.N. staff killed Wednesday were from Ethiopia, Ghana, Liberia, the Philippines and the United States, Siddique said. The nine wounded suffered mostly cuts and bruises as they tried to escape, he said.

Afghan police and U.N. officials said two security guards and the brother-in-law of one of Afghanistan's most powerful governors, Gul Agha Sherzai, were also killed, in addition to the three attackers.

The two-hour assault on the guest house began shortly before 6 a.m. when three gunmen wearing green uniforms and homicide vests broke into the three-story residential hotel. The crackle of gunfire echoed across the city and explosions set fire to the building, filling the lobby and the upper floors with smoke. Terrified U.N. workers scrambled over the roof or leapt from windows to escape.

About a mile away from the guest house, one rocket struck the "outer limit" of the presidential palace but caused no casualties, presidential spokesman Humayun Hamidzada said. Two more rockets slammed into the grounds of the luxury Serena Hotel, favored by foreigners, filling the lobby with smoke and forcing guests and employees to take refuge in the basement.

The election pits President Hamid Karzai against former Foreign Minister Abdullah Abdullah and Washington sees it as crucial to restoring legitimacy to the corruption-riddled government. U.N.-backed auditors threw out nearly a third of Karzai's votes from the Aug. 20 ballot because of fraud, pushing Karzai's totals below the 50 percent threshold needed for a first-round victory.