Suspected militants attacked a police checkpoint in Pakistan's northwest and killed at least four policemen early Friday, police sources said.

Gunmen opened fire at the roadside checkpoint in Mansehra district in the northwest Khyber Pakhtunkhwa province, a police officer said.

One police man was injured, police said.

The gunmen fled after the attack and no group claimed responsibility of the incident.

Suspected militants are active in the area and they had attacked the security personnel, police and foreign-funded NGOs in Mansehra district.

Police said that the security forces launched operation for the search of the militants in the area.

In March, at least five people including two women were killed when armed militants attacked the office of a United States-based NGO in Mansehra.

In February militants launched suicide attacks on two police stations in Mansehra district and killed a police officer and injured 10 other people.