KABUL (Agencies) Taliban suicide gunmen stormed a UN guesthouse in Kabul Wednesday, killing 12 people - including six UN staff - in an attack that the militia said signalled a bloody countdown to new Afghan elections next week. President Hamid Karzai ordered an urgent security upgrade for international organisations after the rampage. The US embassy said one American working for the UN was also killed in the attack. An Afghan Defence Ministry official said the brazen raid was the work of Pakistani Taliban dressed as police who struck the UN-approved Bekhtar Guesthouse before dawn. This is the deadliest day for the UN since we began our engagement in Afghanistan in 2001, a UN spokesman said as head of mission Kai Eide vowed the organisation would not be deterred from its work. This attack will not, I repeat, will not deter the UN from continuing its work to rebuild the war-torn country and to build a better future for the Afghan people, Eide said. We will remain committed to Afghanistan. The Taliban said they had targeted the guesthouse because of the UN role in helping organise the run-off vote. We have said that we would attack anyone engaged in the process and todays attack is just a start, Taliban spokesman Zabihullah Mujahid told Reuters by telephone. The two-hour attack on the guesthouse where some 20 UN election workers were staying sent people running and screaming outside, with some jumping out upper-storey windows to escape a fire that broke out. One American man said he held off the assailants with a Kalashnikov rifle until guests were able to escape. UN spokesman Adrian Edwards said six UN staff were killed and nine other UN employees were wounded in the assault, which began about dawn in the Shar-e-Naw area of the city. Terrified guests fled the building during the assault - some screaming for help and others jumping from upper floors as flames engulfed part of the three-story building. Afghan police and UN officials said 12 people in all were killed, including the UN staff, three attackers, two security guards and an Afghan civilian. Militants also fired rockets at the luxury Serena Hotel, which is popular with diplomats and foreign businessmen. No casualties were immediately reported, Afghan police said. Captain Daoud Safi, an officer in the Afghan defence ministry, was one of the first on the scene and captured its bloody conclusion on his camera phone. They were Pakistani students who came here for suicide training, said Safi as he showed grainy footage of the mangled bodies dressed in police uniforms. Although a police commander said the attackers had been shot dead, one witness said it was clear they had detonated themselves. There is no way they didnt blow themselves up, said the woman, who was allowed to enter the building after the attack. There were intestines everywhere, legs were blown off. I never want to see anything like that again. President Hamid Karzais palace and police said at least one Afghan civilian and three police were also killed. Karzai, who is running against ex-foreign minister Abdullah Abdullah in the run-off, condemned the attack as inhumane. Certainly one of the aims of the Taliban attack today was to show that they are a force that can disrupt the poll, Afghan analyst Qaseem Akhgar said of the run-off.