At least one homicide bomber detonated his explosives in a busy commercial area full of government buildings in Pakistan's main northwest city Thursday, killing four people and wounding six, police and government officials said.

The attack was the second in three days in Peshawar, and the latest in wave of violence that has killed more than 500 people in Pakistan since October. Insurgents are suspected of avenging an army offensive against the Taliban in a northwest tribal region.

The attackers have struck a range of targets, from markets popular with women to security checkpoints. Thursday's attack occurred in a busy sector of Peshawar where buildings housing the state-run airline, a public school and a government insurance company were located.

TV footage from the scene showed shattered glass and debris covering a wide area as security officials flooded the zone.

Mian Iftikhar Hussain, the spokesman for the provincial government, told the ARY news channel that army installations also were nearby, but that he "cannot say for sure what the target was."

Hussain and senior regional minister Bashir Bilour said two homicide bombers were involved. But police official Arif Khan said there was one attacker who walked up to a checkpoint and detonated his explosives when police approached him.

On Tuesday, a suicide bomber set off explosives at the Peshawar Press Club, a brazen attack on the media in what has long been an unsafe environment for journalists to operate.

The government condemned the attacks but vowed that it will not be deterred in its battle to eliminate the Pakistani Taliban from its soil. The army offensive in South Waziristan tribal region has left hundreds of militants dead, but many are believed to have simply fled to other parts of the country's lawless tribal belt bordering Afghanistan.