Pakistan's Taliban claimed responsibility for an attack on a school bus on Tuesday which killed five people, saying the children on the bus were from a pro-government tribe.
The attackers fired a rocket-propelled grenade and AK-47 assault rifles at the vehicle in the northwestern city of Peshawar, killing four children and the driver. Eighteen people, including 15 children, were wounded, police said.
The Pakistani Taliban, seen as the biggest security threat to the government, have become more brazen since their ally Osama bin Laden was killed in a US raid in Pakistan in May.
The militants have carried out high-profile suicide bombings and attacked a major naval base and Western targets.
They have also stepped up pressure on tribes who oppose them.
"Our comrades attacked the bus which was carrying children from the Aka Khel tribe, whose people are fighting against us at the behest of the Pakistani army," Taliban spokesman Mohammed Afridi told Reuters by telephone from an undisclosed location.
"They have been told time and again to desist from any activity against us but they did not listen. We will continue to carry out such attacks."
The Pakistani Taliban are holding hostage more than 20 young men from another pro-government Pashtun tribe in an area straddling the border with Afghanistan and have demanded the release of scores of prisoners and an end support of offensives against them.
The teenage tribesmen from Pakistan's northwestern Bajaur region were abducted by the militants on Aug. 31 while they were on an outing in Afghanistan's border province of Kunar.
At Peshawar's Lady Reading Hospital, children lay in beds with shrapnel and bullet wounds from Tuesday's attack, their uniforms soaked in blood.
"We were in the van, going home like every day. Suddenly I heard an explosion and gunfire," said 8-year-old Sabir.
Pakistan's Taliban, allies of militants in neighbouring Afghanistan, want to topple the US-backed government and impose their strict version of Islam which would involve public whippings and executions for those deemed immoral.
Al Qaeda have suffered several setbacks since bin Laden's death.
But the Pakistani Taliban show no signs of letting up despite a series of army offensives against their strongholds along the unruly mountainous border with Afghanistan.
They plan to kidnap senior Pakistani officials to pressure authorities to release relatives of bin Laden detained after his death, interior minister Rehman Malik said last week.
Bin Laden's wives and several of his children are being held by authorities in the South Asian nation.