Gunmen armed with grenades killed at least three Shia Muslims and wounded six others Monday after ambushing a bus in a Pakistani tribal region plagued by sectarian violence, officials said.

The incident took place in Charkhel village of Parachinar, a flashpoint for sectarian violence between majority Sunni and minority Shia Muslims in Pakistan's tribal area on the Afghan border.

"Gunmen attacked a passenger bus carrying Shia Muslims with hand grenades and firearms, killing three Shias and wounding six others," a local administration official told AFP on condition of anonymity.

The bus came under attack while en route to Peshawar, the main city in the northwest, he said, adding that nobody has so far claimed responsibility.

A security official in the area confirmed the attack and casualties.

Parachinar is the main town in Kurram district, part of the semi-autonomous tribal belt where US drone strikes target Taliban and Al-Qaeda-linked militants in what Washington considers the world's largest international terror hub.

Around 5,000 people have been killed in outbreaks of sectarian violence between Sunni and Shia militant groups in Pakistan since the late 1980s.