Taliban gunmen stormed a private guesthouse in the Afghan capital, Kabul, in a bloody predawn attack Wednesday that killed six United Nations staff and signaled a clear intent to disrupt the upcoming presidential runoff.

The United States embassy in Kabul confirmed that at least one of the dead was an American.

The raid, which appeared to be coordinated with rocket attacks on the presidential palace and the luxury Serena Hotel popular with foreigners, was the worst attack the UN has faced in Afghanistan and, like assaults on aid workers elsewhere could prompt some to leave the country.

The Taliban said the attack was the start of a campaign to wreck a runoff vote scheduled for Nov. 7.

Before dawn three militants disguised as police and wearing suicide vests sealed off the road outside the Bekhtar Guesthouse in central Kabul before shooting their way past a security guard. Once inside, they threw grenades and, by some accounts, dragged guests from their beds before killing them. At least one detonated his vest before security forces recaptured the building following a two-hour siege.

Eyewitnesses said that some terrified guests fled over the roof of a next-door building; others were injured jumping from balconies as flames engulfed part of the building. Twelve people were killed in total, with nine more said to be in serious condition.

Attack could scare off aid workers

Kai Eide, the UN head of mission in Afghanistan, said the attack would not deter the organization from its mission but that there would be a review of security measures. A truck bombing of the UN headquarters in Bagdad in 2003 killed 22 people and prompted the organization to pull out of Iraq for several years. Although UN workers have occasionally been targeted in Afghanistan before, Wednesday’s attack had the highest death toll so far.

The attack could have a heavy impact on aid agencies in the country, with security analysts warning that some may pack up altogether while others will definitely scale back their operations. Aid workers due to arrive in the country to help with the second round of voting have been advised to delay their flights.

“I think there’s going to be a bit of an exodus,” says one analyst who asked not to be named.

Aid workers are softer targets than the coalition bases, embassies and government ministries that the Taliban has concentrate its attacks on in the capital up til now.

President Hamid Karzai ordered an urgent overhaul of security around international aid institutions.

Deadliest month since 2001

The attack came as the loss of eight more American soldiers on Tuesday brought total troop casualties this month to 54, making October the deadliest month of the war for US forces since it began in 2001. The casualty rate has shot up sharply since July when thousands of additional US troops were deployed to some of the most volatile parts of the country.

Seven soldiers fell victim to a cluster of roadside bombs in southern Afghanistan, while an eighth was killed in an explosion in another part of the country. Improvised explosive devices, the Taliban’s weapon of choice, are the largest single killer of foreign forces in the country. (Read here about how US troops are trying to counter the IED threat in Afghanistan.)

The rising number of casualties, the high-profile resignation of a highly regarded US Foreign Service officer, and the growing tensions over next week’s Afghan election runoff will intensify pressure on Barack Obama as he edges towards a crucial decision on whether to commit thousands more troops to Afghanistan.