At least nine people were killed when a suicide bomber detonated outside a hospital in north-western Pakistan on Sunday. Another 26 others were wounded, local officials said. The Tehrik-e-Taliban Pakistan group claimed responsibility for the attack on a hospital in Dera Ismail Khan early on Sunday morning. The group is often known as the Pakistani Taliban, although it is separate from the insurgent movement across the border in Afghanistan. Local officials said the attack happened after two police were killed at a roadside checkpoint outside the city. The bomber was reportedly wearing a burqa but the Pakistani Taliban did not claim the attacker as a woman Inayat Ullah, a local forensics expert, said the female attacker set off 7 kilograms (15 pounds) of explosives packed with nails and ball-bearings. Salim Khan, a senior police officer in Dera Ismail Khan, said gunmen on motorcycles opened fire on police in a residential area, killing two. He said the bomber then struck at the entrance to the hospital, killing another four policemen and three civilians who were visiting relatives. He said eight policemen were among the wounded, and that many of those injured were in critical condition. Inayat Ullah, a local forensics expert, said the attacker set off 7 kilograms of explosives packed with nails and ball-bearings. The blast damaged the emergency room and forced it to shut down, according to a hospital official, who said the wounded were taken to a military hospital. The official spoke on condition of anonymity for fear of reprisal. "When their bodies were taken to the main hospital, a suicide bomber, believed to be a female, wearing a burqa, triggered an explosive-laden jacket and caused huge losses," said Waqar Ahmad, a local police officer. He said the blast had damaged the emergency ward of the hospital and forced some of the wounded to be transferred to other cities. Muhammed Khurasani, a spokesman for the TTP, claimed responsibility for the attack, which he said was carried out in revenge for the killing of two Taliban commanders by counter-terrorism police almost a month ago. However, he denied the bomber was female. The group has launched scores of attacks going back nearly two decades, but almost all of them were carried out by men Dera Ismail Khan has seen a number of suicide attacks over the past decade as the Pakistani military carried out a campaign to suppress militant attacks in the area, an important hub in the $60 billion China Pakistan Economic Corridor.