Smoke comes out of a guest house used by U.N. staff after an attack by gunmen in Kabul, Afghanistan today. A Taliban spokesman claimed responsibility, saying it was meant as an assault on the upcoming presidential election.

KABUL — Taliban militants wearing suicide vests and police uniforms stormed a guest house used by U.N. staff in the heart of the Afghan capital early today, killing 12 people — including six U.N. staff. It was the biggest in a series of attacks intended to undermine next month's presidential runoff 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 chief of the United Nations mission in Afghanistan, Kai Eide, said the attack "will not deter the U.N. from continuing all its work" in the country. One of the six U.N. dead was an American, the U.S. Embassy said.

The two-hour attack on the guest house where some 20 U.N. 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 the Taliban issued a statement threatening anyone working on the Nov. 7 runoff election between Karzai and Abdullah Abdullah.

"This is our first attack," he said.

Witnesses said the attack started shortly before 6 a.m. 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 organizations to be alert to possible suicide attacks.