ISLAMABAD: Pakistan's army has sent home two-thirds of the US military personnel who were training its forces in counterinsurgency skills along the porous border with Afghanistan.

A senior Pakistan military official said late Tuesday that 90 of an estimated 135 US trainers have left the country, the latest setback in the deeply troubled relationship between the United States and Pakistan's military following the May 2 US raid that killed al Qaeda leader Osama bin laden.

The 90 Americans had been training the Frontier Corps, a paramilitary force made up mostly of tribesmen from the frontier areas, according to the military official, who asked not to be named in accordance with military practice.

The Frontier Corps is Pakistan's front line force against militants in the tribal regions. The US military personnel were teaching members of the force to become trainers.

The US has confirmed it is reducing the number of its military personnel in Pakistan but has not given an exact figure.

''We have reassessed our requirements and sent 90 people home,'' said the Pakistani military official. Other Americans have also been ordered to leave Pakistan, but the official would not elaborate or provide details.

''Where essential elements are required we are keeping them. In very critical areas of maintenance and technical capability, where we do not have the qualified people then we are keeping them,'' he said. ''But otherwise they are being asked to leave.''

Washington's relationship with Pakistan has been shaky for months. Pakistan first requested a withdrawal of US forces after the arrest and detention in January of CIA security contractor Raymond Davis, the official and Western diplomats have said.

Davis was arrested for the shooting deaths of two Pakistani men, who he said were trying to rob him. He was eventually released in March after the dead men's relatives agreed to accept blood money under Islamic tradition.

The bin Laden raid worsened relations and escalated the drawdown of US personnel in Pakistan.