That US forces regularly use unmanned drones to attack terrorist targets in Pakistan is nothing new, but a report at the Guardian suggests that US special forces repeatedly put "boots on the ground" inside the country as part of the war against insurgents.

Citing a "former NATO officer" with "detailed knowledge of the operations," the Guardian reported late Monday evening that the US launched "multiple clandestine raids" into Pakistan between 2003 and 2008, and that the Pakistani government was not informed of the raids.

The unnamed NATO source details four separate incidents in which US troops landed on Pakistani soil, including the only previously reported raid: A September, 2008, operation which targeted three houses inside Pakistan, near the Afghanistan border, which reportedly killed 15 people.

Of the other three operations, two were attacks on suspected militants (one of which failed), and the third was a rescue operation to retrieve a Predator drone. The fact the US military would send in a clandestine force to extract the drone, rather than ask the Pakistani military for help, shows how uneasy the alliance between the US and Pakistan is, the Guardian reports.

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.

In recent months, news has been coming out about the extent of the US's military involvement in Pakistan. Last month, The Nation reported that the Obama administration is using the controversial security contractor Blackwater to kidnap or kill "high-value targets" in Pakistan.

Erik Prince, the CEO and founder of the security firm, appeared to confirm his own deep involvement in the CIA's role in the war on terrorism, telling Vanity Fair that he had been recruited as a CIA "asset."

Read the complete Guardian report here.