(Adds second attack, details)
By Mirwais Harooni
KABUL, Jan 21 (Reuters) - A coordinated attack involving at least three suicide bombers and a powerful car bomb took aim at the headquarters of the Kabul traffic department on Monday, followed by a clash between at least one insurgent and security forces, police said.
The attack took place just days after six suicide bombers attacked the Afghan spy agency in Kabul, killing two.
The Taliban claimed responsibility for Monday's attack.
"Today at 5 o'clock in the morning (0130 GMT) a number of mujahideen martyrs entered a government building close to an American training centre... Heavy fighting is ongoing," spokesman Zabihullah Mujahid said in a text message to media.
Police said it was not immediately clear if there were any casualties in the attack, which involved a second bombing, a tactic favoured by Islamist insurgents elsewhere but relatively rare in Afghanistan.
"About an hour after the initial attack (triple suicide bomb attack) a fourth man drove a car to the same compound and detonated another bomb," the head of the Kabul police criminal investigation department, Mohammad Zahir, told Reuters.
Violence across the country has been increasing over the last year, sparking concern over how the 350,000-strong Afghan security forces will be able to manage once foreign troops withdraw by 2014.
(Writing by Amie Ferris-Rotman; Editing by Nick Macfie)