The hostages were released because of pressure on the abducters from tribesmen in the area, he said.

"They released them on their own. We don't know their motive nor did they demand anything from us," Arif said.

One of the released workers declined to comment.

"We don't have permission to comment. First, we'll meet our seniors and then they will tell you what you want to know," Zair Mohammad Wazir, one of the released worker, said.

Many foreign fighters took refuge in the North and neighbouring South Waziristan after US-led forces toppled the Taliban regime in Afghanistan in 2001.

Pakistani security forces launched offensives in the region in 2003 as part of the US-led war.

But the government, in a bid to isolate the fighters, later struck peace deals aimed at enhancing traditional powers of leaders of the ethnic Pashtun tribes who inhabit both sides of the border.