Pakistan's army offensive in tribal areas bordering Afghanistan heated up Monday, with militants attacking three security force bases and military jets responding with airstrikes that killed at least 21 people, intelligence officials said. The overnight and early morning clashes follow artillery attacks Sunday on suspected militant hideouts in two towns in the northwest that killed 27 fighters, officials said. Elsewhere in the volatile region, a citizens' militia killed seven suspected militants. Meanwhile, in an apparent response to the military action two policemen were killed Monday when a suicide bomber blew up explosives packed in his vehicle in Pakistan's northwest, where the military are battling Taleban rebels, police said. The attack happened just on the border of Mansehra district and Swat, a scenic valley region where the military are currently locked in an eight-week offensive to crush Taleban militants in the area. “An explosion occurred in a small truck when it reached the Dandai bridge in Shangla district (of Swat) ... It was a suicide attack,” said Shangla district police chief Gul Wali Khan. “Two policemen were martyred and three others were injured in the bombing.” Pakistan has been hit by a wave of deadly suicide bombs in recent weeks, attacks blamed on Taleban rebels seeking to avenge the campaign against them. The army confirmed last week that the offensive would be expanded into the lawless tribal areas along the Afghan border, the stronghold of feared Pakistan Taleban chief Baitullah Mehsud and his fighters. The government announced last week that the military would go after Pakistan's Taleban commander, Baitullah Mehsud, in the South Waziristan tribal area. His stronghold is a chunk of the remote and rugged mountainous region where heavily armed tribesmen hold sway and Al-Qaeda and Taleban leaders are believed to be hiding. Militants used mortars, rockets, gunfire and even an anti-aircraft gun to attack three military bases overnight in South and North Waziristan, five intelligence officials said, speaking on condition of anonymity because they are not authorized to talk with media. Their reports could not be confirmed because of a lack of media access to the conflict zones, and official military spokesmen could not be reached for comment. While most of the dead appeared to be militants, three women and three children died when the house of a local tribal leader was hit in the Razmak area, one official said. Qari Hussain, a close aide of Mehsud, telephoned The Associated Press on Monday to say the military strikes had not weakened the Taleban in South Waziristan, and claim they hit civilians and destroyed their homes.