Security officials in southern Afghanistan are warning international organizations to be on the alert in the wake of a deadly attack in the capital, Kabul.

Taliban militants wearing suicide vests and police uniforms stormed a guest house used by U.N. staff in the heart of the capital early today, killing 12 people — including six U.N. staff. One of the staffers was an American. 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.

President Hamid Karzai condemned the attack as "an inhuman act."