A suicide bomber rammed an explosives-laden vehicle into a police station in Pakistan's restive Swat Valley early Thursday, killing four people and destroying the building, police said.
   
The attack was the latest in a string of bombings on security forces in the northwest of Pakistan, where the government is waging a military campaign against Al-Qaeda and Taliban-linked militants near the border with Afghanistan.
   
"It was a huge blast. Three security force personnel and one policeman were killed. The police station was destroyed," Mingora city police chief Dilawar Khan Bangash told AFP.
   
He said the co-ordinated attack had begun soon after midnight with militants and security forces trading fierce gunfire before the suicide bomber drove a 14-seater bus into the back of the police station's grounds.
   
The vehicle exploded as it came under fire, damaging scores of shops, a hotel, a school and many houses.
   
Most of the 30 people injured were security forces and police, though two by-standers were also hurt, he said.
   
The police station in Mingora, the main town of the Swat Valley, was also attacked by militants in July, when two members of the security forces were injured.
   
The building was destroyed in Thursday's attack, with its heavy concrete floors blown apart and its walls reduced to rumble.
   
Pakistan's government has tried to enlist fiercely independent ethnic Pashtun tribesmen in the northwest to back its military operations against Al-Qaeda and Taliban extremists.
   
But the militants have fought back, killing dozens of security forces and also many tribal elders they accuse of backing the government.
   
The luxury Marriott hotel in the capital Islamabad was hit by a massive suicide bomb on September 20 when bomber drove a truck containing 600 kilogrammes (1,300 pounds) of high explosives into its outer gates.
   
At least 60 people were killed in the blast, which officials suspect was carried out by Osama bin Laden's Al-Qaeda network.
   
The United States has become increasingly concerned at Pakistan's failure to tackle Taliban militants based in the lawless tribal belt and in the adjoining North West Frontier Province bordering Afghanistan.
   
The incident comes after a string of US missile attacks into Pakistan, and one ground incursion by US troops deployed in Afghanistan, which have raised tensions between Islamabad and Washington.