PESHAWAR: Pakistani security forces killed 18 Taliban militants on Thursday in Bajaur region, a military official said, in the fourth straight day of fighting which has cast doubt on the effectiveness of crackdowns on militant groups.

Pakistan's military, one of the largest in the world, has launched a series of offensives against the Taliban over the past few years in Bajaur and other parts of the ethnic Pashtun tribal belt in the northwest near the Afghan border.

On Wednesday, Taliban militants attacked an army post and killed eight soldiers in the South Waziristan tribal region.

Military sources said four soldiers have been killed in the Bajaur fighting and several went missing.

Pakistan has said military operations have severely weakened the militants, and their fighters have been cleared out of Bajaur and other regions along the porous border.

The military, which has declared victory in those areas, said the militants killed on Thursday included two commanders from the Taliban branch that controlled the Swat Valley, before an army offensive cleared the group from there in 2009.

Their presence in Bajaur underscores how militants simply melt away when the military steps up pressure and turn up elsewhere to press on with their violent campaign to topple the US-backed government.

The commander who led the Taliban in Swat, Fazlullah, fled to Afghanistan and has been orchestrating cross-border raids against Pakistani security forces, sending hundreds of fighters at a time to stage deadly ambushes and attacks on army posts.

This month, militants armed with rocket-propelled grenades and automatic weapons fought their way into one of Pakistan's largest air bases in a brazen challenge to the nuclear-armed country's powerful military.

Pakistan's Taliban, who are close to al Qaeda, allied with the Afghan Taliban and seen as the biggest security threat to Pakistan, claimed responsibility for that assault.