Ambassador Cameron Munter emphasised in a news conference at the US embassy that Pakistan's tackling of the militant sanctuaries on the Pakistan side of the border - considered crucial for stabilising Afghanistan - was a matter of capacity and not of will.

“Yes, I believe that,” he said when asked whether the United States believed the Pakistanis were sincere about going after the safe havens used by militants for years as places to rest and re-arm after carrying out attacks on Western forces.

Munter spoke a day after the White House unveiled a review of President Barack Obama's year-old strategy to pour an additional 30,000 US troops into Afghanistan in an effort to check Taliban momentum and allow for a gradual drawdown of combat troops starting in mid-2011.

A five-page unclassified summary of the review said foreign forces had made “notable operational gains” in Afghanistan, but reported uneven progress in Pakistan, whose border areas are widely seen as the main obstacle to Obama's strategy succeeding because of the free flow of militants into Afghanistan.

Pakistan has about 140,000 troops on its western border with Afghanistan involved in several operations against Pakistani militants bent on overthrowing the government in Islamabad.

But Pakistan has been reluctant to go into North Waziristan, often called the epicentre of global jihadism, because it says it needs to consolidate gains made elsewhere before it can tackle the rugged territory that is thought to be a base for some members of the Afghan insurgency and al Qaeda.

“There is a great amount of capacity being used at holding the ground the Pakistani Army has won at great cost,” said Munter.

US Defense Secretary Robert Gates said on Thursday that Pakistan needed to do more to control the flow of extremists along its porous border with Afghanistan. Pakistan says it will invade North Waziristan, but on its own schedule.

On one hand, officials in Washington say Pakistan has to do more to go after groups such as the Haqqanis. On the other, Munter and other officials are publicly reluctant to press too much because Pakistan could make life difficult for Nato, as it did in October when it closed down a vital supply route for 10 days because of a coalition helicopter incursion that killed two Pakistani troops.

“We would like them to move tomorrow, we would like them to take out these people tomorrow,” said Munter. “But we understand they're telling us honestly about the capacity of their military, and when they are able, we are convinced they will move in.”