Britain is considering sending special forces troopers to help train a Pakistani militia army in the Taliban's tribal heartland of Waziristan.

The controversial move, neither confirmed nor denied by the British embassy in Islamabad, follows a US plan to send up to 100 special operations soldiers to hone the skills of the Frontier Force, a paramilitary organisation drawn from the ethnic groups along the border with Afghanistan.

Ministry of Defence policy is not to discuss the deployment of SAS or SBS operatives, but an insider confirmed a training mission which might have a real impact on insurgent recruitment and sanctuary on the Pakistani side of the border was under discussion.

The main problem, he added, was where to find the skilled manpower from the UK's overstretched special forces, already fully committed in Iraq and Afghanistan.

The SAS and SBS have fewer than 600 "shooters" between them and have been working back-to-back tours since 2001, moving tired troopers between operational squadrons to allow them brief breaks from operations.

The aim of the mission would be to bring units of the 85,000-strong Frontier Force up to a level where they could defeat Taliban and foreign jihadi groups operating against coalition troops in Afghanistan from the relative safety of Pakistan's tribal territories.

American advisers have been quietly helping to train Pakistani regulars in counter-insurgency tactics, but the new plan, presented in the Pentagon, would involve the very tribal manpower pool which is exploited by the Taliban.

The scheme is also understood to be aimed at providing Pakistan's new government with a "minimum risk for maximum potential return" policy.

The Pakistani army has lost more than 800 men in the tribal lands in three years and Islamabad's hold on the fiercely independent area remains fragile.

Under the joint US/UK plan, training Pashtun and Pathan militiamen to keep the insurgents in check would not only ease that burden, but might also divert the militants from their aim of launching a terror campaign to undermine the fledgling government.

With relative handfuls of Western troops involved, there would also be less risk of inflaming Pakistani sensitivities over the presence of armed foreigners on their soil.

A US source said the plan had not yet been formally approved, but that "promising" negotiations were under way with the leaders of the victorious Pakistan People's Party.

While the trainers would initially be confined to base, it is understood that once their presence became accepted, they might accompany Frontier Force units "to the point of contact" with militants.

In Afghanistan yesterday, a suicide car bomber killed a policeman and injured four others in a government building in Khost province, near the Pakistan border.

The attack came a day after Taliban insurgents attacked a joint Nato and Afghan military compound, then rammed a lorry bomb into the base. Two Nato soldiers were killed and 15 people were wounded.