Authorities say a roadside bomb has killed an anti-Taliban militia member and three others in Pakistan's northwest.

Police say the improvised explosive device targeted the vehicle of Aziz ur-Rehman in the Lower Dir district of Khyber-Paktunkhwa province, which borders the country's semi-autonomous tribal belt on the Afghan border.

Rehman, his 12-year-old son and two others were killed in the blast. At least two others were also wounded.

It is not immediately clear who is responsible for the attack. Pakistan's northwest tribal belt is a known stronghold for al-Qaida and Taliban-linked militants.

Pakistan has long encouraged the formation of local militias, known as lashkars, to help in the fight against the Taliban, especially in remote areas near the Afghan border. Members of the lashkars are often the targets of militant violence.

Elsewhere in Pakistan's northwest, thousands of people continue to flee an ongoing military offensive against militants in the Khyber tribal agency.

The army operation was launched last week after militants killed nine paramilitary soldiers in an ambush.

Officials said Tuesday that at least 3,000 displaced families have been registered at the Jalozai camp in the Nowshera district of Khyber-Pakhtunkhwa province. The families are seeking shelter and other assistance after they were evacuated from their homes by security forces.

The fighting has so far killed at least three Pakistani soldiers and 34 militants.