Taliban militants wearing suicide vests and police uniforms stormed a guest house used by UN staff in the heart of the Afghan capital early today, killing 12 people - including six UN staff.
It was the biggest in a series of attacks intended to undermine next month's presidential run-off election.
A Taliban spokesman claimed responsibility for the early morning assaults, which also included rocket attacks at the presidential palace and the city's main luxury hotel.
The head of the United Nations mission in Afghanistan, Kai Eide, said the attack "will not deter the UN from continuing all its work" in the country. One of the six UN dead was an American, the US Embassy confirmed.
The two-hour attack on the guest house where some 20 UN election workers were staying sent people running and screaming outside, with some jumping out upper-story 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.
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 Serena Hotel, which is favored by many foreigners.
One failed to explode but filled the hotel lobby with smoke, forcing guests and employees to flee to the basement, according to an Afghan witness who asked that his name not be used for security reasons.
President Hamid Karzai condemned the attack as "an inhuman act" and called on the army and police to strengthen security around all international institutions.
Taliban spokesman Zabiullah Mujahid claimed responsibility for the attacks in a telephone call to The Associated Press, saying three militants with suicide vests, grenades and machine guns carried out the guest house assault.
He said three days ago that the Taliban issued a statement threatening anyone working on the November runoff election between Karzai and Abdullah Abdullah.
"This is our first attack," he said.
An official with the UN election team said that the Bakhtar guest house was home to the largest concentration of UN election workers in the city. The official did not give a specific number of election workers staying there, but said it was about 20.
Witnesses said the attack started shortly before 6.00am and lasted for about two hours. Interior Ministry officials said the attackers were wearing old-style police uniforms, which are available in markets. In the southern city of Kandahar, security officials also warned international organisations to be alert to possible suicide attacks.
UN spokesman Adrian Edwards said six UN staff were killed and nine other 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. The bodies of the attackers were taken out of the house and sent for autopsies, said Gul Mohammad, an officer at the scene.
It was not immediately known how the victims were killed or how the fire started, but witnesses said they heard prolonged gunfire ringing from the house before police arrived at the scene. It also was not immediately clear whether there were any other attackers besides the three killed.
Police were seen pulling the charred body of what appeared to be a woman from a second-floor bedroom. One officer carried an injured German man by piggyback away from the scene.
Terrified guests scrambled over the roof or jumped from windows to escape.
John Turner, a trucking contractor from Kansas City said he held off attackers with a Kalashnikov until a group of guests escaped through the laundry room. Miles Robertson, an Australian working as an election adviser, said he and his wife put wet towels over their faces and fled when the room next door caught fire.
Mr Edwards said officials were trying to account for several other UN workers who were staying at the guest house. He did not know their nationalities but said they were non-Afghans.
"This has clearly been a very serious incident for us," Mr Edwards said. "We've not had an incident like this in the past."
He said the UN would have to evaluate "what this means for our work in Afghanistan." The truck bombing of the UN headquarters in Baghdad in August 2003, which killed 22 people, prompted the UN to pull out of Iraq for several years.
AP