At least 18 militants were killed, four others arrested during a clash with security forces on Friday in the northwest of Pakistan, official sources said.

The exchange of firing took place as security forces conducted search and clearance operation in Orakzai tribal agency.

According to official sources, the troops destroyed three hideouts of militants and captured large quantity of arms and munitions.

A security man was also injured in the fighting.

The Orakzai offensive has become intensive since late March, in which over 300 militants have been killed or injured.

Asad Khan, a local journalist based in Hangu, a district near Orakzai, told Xinhua that above 80 percent people have left the area and moved to Hangu and Kohat cities as security forces have warned them to evacuate the area.

Large numbers of militants fled from South Waziristan tribal area have arrived in Orakzai months ago before the military operation was started in the stronghold of Tehrik-e-Taliban Pakistan (TTP).

Orakzai agency is a mountainous and hilly region with a fertile valley. In this regard it is a paradise for al-Qaeda and other militants to find sanctuaries and use it as the launching pad against Pakistani security forces and even for Afghanistan violence.