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 Wednesday, 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.