By Mirwais Harooni and Hamid Shalizi

KABUL (Reuters) - Suicide bombers and gunmen attacked the headquarters of the Kabul traffic police on Monday, local officials said, in the second coordinated attack on a government building in less than a week.

The Taliban claimed responsibility for the attack, which raised the possibility that insurgents were now more focused on testing Afghan security forces in Kabul after a series of high-profile attacks on Western targets last year.

Violence across the country has been increasing over the last year, sparking concern about how the 350,000-strong Afghan security forces will be able to manage once foreign troops withdraw by 2014.

Four policemen and seven civilians were wounded in the assault, which began around 6 am (0230 GMT) when two bombers detonated explosives, said Interior Ministry spokesman Sediq Sediqqi.

Up to two more suicide bombers armed with automatic rifles then stormed the building, nestled between two police hubs and not far from parliament and the adjoining road that is frequented by Afghan lawmakers on their way to work.

Thick black smoke rose from the besieged compound as an Afghan Army helicopter hovered above the site where insurgents and security forces engaged in a fierce firefight.

"Honestly speaking, this type of attack, at the start of the year, indicates the coming months are going to be tough," a government official said, speaking on condition of anonymity.

"The Taliban will want to display their presence and reach with these kinds of attacks in Kabul."

Last week, a band of six suicide bombers attacked the National Directorate of Security (NDS), killing two of its guards. That attack followed a failed assassination attempt on NDS chief Asadullah Khalid.

(Writing by Amie Ferris-Rotman and Dylan Welch; Editing by Nick Macfie)