The sensitive mission puts rare American boots on the ground in a key theater in the war against extremist groups, but it risks fanning anti-U.S. sentiment among Pakistani Muslims already angry over suspected CIA missile attacks on militants in the same frontier region.
"The American special forces failed in Afghanistan and Iraq," said Ameerul Azim, an official in the hard-line Islamic party Jamaat-e-Islami. "Those who failed everywhere cannot train our people."
Despite such complaints, the training program comes as some tribes in the frontier zone are setting up militias to help the Pakistani government combat extremist movements. The new forces have been compared to the Sunni Arab militias in Iraq that helped beat back the insurgency there.
Still, the U.S. training program is reportedly smaller than originally proposed and was delayed, apparently reflecting misgivings in Pakistan's government about allowing U.S. troops on its territory.
Its start has not been officially announced, but a Pakistani military officer and a U.S. defense official told The Associated Press that two to three dozen trainers arrived earlier this month.
The Pakistani said the Americans had already begun training senior personnel of the paramilitary Frontier Corps at an undisclosed location in Pakistan's restive northwest, adjacent to Afghanistan. He said the course included classroom and field exercises.
The Pentagon official said the Americans would stay for a few months. He said that it would likely be a one-time effort and that there were no plans to send more trainers.
Both officials agreed to discuss the program only if granted anonymity, because details had not been made public.
Asked about the program Thursday, Pentagon spokesman Bryan Whitman declined to give any specifics. But he contrasted the mission with much larger U.S. training efforts in Iraq and Afghanistan, where U.S. soldiers are embedded with local units on the battlefield.
"It is a train-the-trainer type of concept," Whitman said. "They are not actually conducting operations."
The Frontier Corps is a relic of British rule that was long a poorly armed, untrained police force that the government hopes can be remade into a potent unit capable of confronting Taliban militants.
Its troopers are local men, in contrast to the army, which is dominated by ethnic Punjabis and is viewed as an occupying force by the Pashtun tribes living on both sides of the border. U.S. and Pakistani officials argue that the corps' local knowledge and cultural sensitivities make it the best tool in a battle where winning hearts and minds is crucial.
The goal is that a strong Frontier Corps can take on most combat duties, allowing a gradual pullback of the army that is hoped will ease tensions in the northwest.
The U.S. has poured some US$10 billion into Pakistan since the then-president, Gen. Pervez Musharraf, turned against his former Taliban allies in Afghanistan after the Sept. 11 attack on the U.S. Most of the money has gone to the army, including the US$70 million earmarked for the Frontier Corps program.
U.S. forces already trained Pakistan's Special Services Group, a commando unit that crushed militants holding Islamabad's Red Mosque last year. Washington also has supplied the helicopter gunships that are seeing heavy use in army offensives in several Pakistani border regions.
But with the war dragging in Afghanistan, U.S. lawmakers and commentators have questioned why Pakistan still seems unable to eradicate militant sanctuaries on its side of the border.
"This thought has come pretty late in the day," Rasul Bakhsh Rais, a professor of political sciences, said of Pakistan's decision to let the trainers in. "But still, I don't think it is too late, given the fact that this is going to be a very long war."
With many Pakistanis accusing their army of fighting a proxy war against its own citizens at Washington's behest, U.S. officials have said Pakistan was reluctant to accept foreign training, but softened its stance in the light of mounting losses.
Musharraf, who was forced out of office earlier this year, announced a plan in 2007 to build up the Frontier Corps so it could confront Taliban fighters.
At the time, its troops had no body armor, few vehicles and an arsenal of only aging rifles. With U.S. help, the corps has received several more battalions, been armed with tanks and artillery and is now heavily involved in fighting in the Bajur and Swat areas.
American officials have said they are also supplying equipment such as helmets, flak vests and night-vision goggles.
"The hope is that the more trainers we train, the more effective they will be in training their forces and the more capable forces will then be able to take the fight to the militants in the tribal areas where they operate," Pentagon spokesman Geoff Morrell said.
The training program has begun despite strains in Pakistani-U.S. relations.
Gen. Ashfaq Parvez Kayani, who replaced Musharraf as army chief, and the former leader's successor as president, Asif Ali Zardari, have maintained close ties with Washington. But they have condemned the recent U.S. missile strikes, the latest of which killed nine people Thursday.
Cooperation has also been chilled by an incident in June when U.S. warplanes killed 11 Frontier Corps troopers at a border post. U.S. officials said the action during a skirmish with militants was justified. Pakistan's army insists no shots were fired from the post.
U.S. officials suspect some Frontier Corps troops sympathize with the Taliban and ignore militants sneaking though mountain passes into Afghanistan to attack U.S. and NATO troops.
Pakistani officials agree the corps has problems, but analysts say a better trained force is more likely to have the confidence to take on the militants. American officials also hope it will become a better partner for cross-border cooperation.