Gunmen in northwestern Pakistan attacked a school bus Tuesday, killing four children and the driver and wounding at least 16 others, most of them children.

Local authorities said the attack took place near the city of Peshawar, close to Pakistan's border with Afghanistan. They said militants first launched a rocket at the school bus and missed, then opened fire.

The Taliban claimed responsibility for the attack, saying the children on the bus were from a local tribe that has been fighting insurgents in the region. A spokesman warned that Taliban militants will continue to carry out similar attacks.

U.N. Secretary-General Ban Ki-moon condemned the attack, calling it an “abhorrent act of terrorism.” He extended his condolences to the families of the victims.

The U.N. Children's Fund representative in Pakistan, Dan Rohrmann, described the attack on young children on a school bus as “deeply distressing.” He said that such attacks violate children's right to education and their right to develop to their full potential.

Also Tuesday, the head of the main Awami National Party in northwest Pakistan, Sher Khan, was killed in a roadside bombing near the Swat Valley. His driver and guard were wounded in the attack.