American special forces have conducted multiple clandestine raids into Pakistan's tribal areas as part of a secret war in the border region where Washington is pressing to expand its drone assassination programme. A former Nato officer said the incursions, only one of which has been previously reported, occurred between 2003 and 2008, involved helicopter-borne elite soldiers stealing across the border at night, and were never declared to the Pakistani government. The Pakistanis were kept entirely in the dark about it. It was one of those things we wouldn't confirm officially with them," said the source, who had detailed knowledge of the operations. Such operations are a matter of sensitivity in Pakistan. While public opinion has grudgingly tolerated CIA-led drone strikes in the tribal areas, any hint of American "boots on the ground" is greeted with virulent condemnation. After the only publicly acknowledged special forces raid in September 2008, Pakistan's foreign office condemned it as "a grave provocation" while the military threatened retaliatory action. The military source said that was the fourth raid of previous years. The secretive nature of the raids underscores the suspicious nature of the relationship between the two allies as they argue about Washington's latest demands.