HERAT  - Gunmen dressed in Afghan police uniforms and wearing suicide vests stormed a government compound on Thursday, killing seven people and wounding 12 others in the southwest, police said.The four attackers targeted the offices of Governor Mohammad Ikhpolwak in Farah, capital of the southwestern province Farah, which borders Iran and is considered a trouble spot for the decade-old Taliban insurgency.The assault underscored the major security challenges as Afghanistan heads to a NATO summit in Chicago on Sunday with a demand for $4.1 billion a year to bankroll its security forces after Western troops pull out in 2014.“Four suicide bombers in Afghan police uniforms armed with suicide vests and rocket-propelled grenades today attacked a government compound in Farah, exchanging fire with police,” said Mohammad Ghaus Malyar, provincial deputy security chief.“As a result, seven people were killed - six police and one civilian - and 12 others were wounded including three policemen and nine civilians,” Malyar added.Officials said the four attackers also died.Two of the attackers detonated their suicide vests and the other two were shot dead by police, said interior ministry spokesman Sediq Sediqqi.The ministry blamed the attack on “terrorists”, a phrase Afghan officials use to refer to Taliban insurgents and other militants.The Taliban frequently target government compounds in attacks carried out by multiple militants carrying suicide vests, rockets and machine-guns.In mid-April, dozens of people were killed and injured when Taliban suicide attackers occupied construction sites in Kabul, including one at the heart of the diplomatic enclave, unleashing a battle with security forces that lasted about 19 hours.Farah is a volatile area where remnants of the Taliban ousted from power in the 2001 US-led invasion are fighting to bring down the Western-backed government. This month the militia announced the start of its annual spring offensive, a campaign of bombings and attacks. They have attacked foreign missions in Afghanistan.