A controversial new US tactic to mount counter-terrorist operations inside Pakistan has met with fresh hostility, it emerged yesterday, as Pakistani tribesmen representing half a million people vowed to switch sides and join the Taliban if Washington does not stop cross-border attacks by its forces from Afghanistan.

Reacting to American missile attacks in north Waziristan last week, which followed an unprecedented cross-border ground assault earlier this month, tribal chiefs from the area called an emergency meeting on Saturday.

"If America doesn't stop attacks in tribal areas, we will prepare a lashkar [army] to attack US forces in Afghanistan," tribal chief Malik Nasrullah announced in Miran Shah, north Waziristan's largest city. "We will also seek support from the tribal elders in Afghanistan to fight jointly against America."

The development threatens to widen the conflict, with previously moderate people from Pakistan's tribal border region with Afghanistan in danger of joining Taliban militants based in the area. They have reacted furiously to intensified American missile attacks on targets in the tribal territory in recent weeks.

The issue is likely to feature in talks between Gordon Brown and Pakistan's new president, Asif Ali Zardari, this week. Zardari, who is on a private visit to Britain, is due to meet Brown tomorrow. The prime minister is likely to press for greater Pakistani action against militants in the tribal area and may go along with US calls to integrate the tribal territory into the conflict in Afghanistan as one theatre of war, an idea Pakistan will fiercely resist.

Zardari and Pakistan's prime minister, Yousaf Raza Gilani, said in a joint statement at the weekend: "The sovereignty and territorial integrity of the country should be respected at all cost." During the past month, there have been seven US missile strikes in the tribal area, about the same number as in the whole of last year. A US ground assault in south Waziristan provoked a sharp rebuke from the Pakistan army.

Washington believes that Taliban and al-Qaida militants fighting the western coalition in Afghanistan are using Pakistan's tribal area as a safe haven.

But Ayaz Wazir, a retired Pakistani diplomat who is a tribal chief from south Waziristan, warned: "If the Americans are coming to sort it out with force, they would create more enemies. The Americans might have supersonic jets and we might have to fight with stones in our hands, but we will stand up."

Up to now, only a tiny minority of the tribesmen have joined the Pakistani or Afghan Taliban movements, but incursions by the US could ignite the area.

The heightened US activity comes just as some Pakistani tribes have risen against the Taliban in the border areas of Dir and Bajaur. But hatred of America would far surpass any dislike for Islamic extremists.