The pre-dawn helicopter-borne ground assault on the village of Angor Adda in the South Waziristan region on the Afghan border on Wednesday, was the first known incursion into Pakistan by US-led troops since the invasion of Afghanistan in 2001.

The United States says al-Qaeda and Taliban militants are based in sanctuaries in northwest Pakistan's ethnic Pashtun tribal areas on the Afghan border, where they orchestrate attacks in Afghanistan and Pakistan and plot violence in the West.

While Pakistan is a US ally in the unpopular campaign against terrorism, it rules out incursions by foreign troops into its territory.

There have, however, been numerous missile strikes on militants in Pakistan, most believed launched by US-operated pilot less drone aircraft.

US Ambassador Anne Patterson was summoned to the Foreign Ministry on Wednesday evening after Pakistan strongly condemned the raid as a gross violation of Pakistani territory that could undermine security cooperation, a Ministry spokesman said.

"She said she would convey our concerns and the protest of the government of Pakistan to her government," said Ministry spokesman Mohammad Sadiq.

A US embassy spokesman declined to comment.

While there is little, if any, doubt the raid was carried out by US troops, it was not clear which Afghan-based force they came from.

‘NOTHING FOR YOU’

NATO, which commands an Afghan peacekeeping force known as the International Security Assistance Force (ISAF), denied any involvement.

"I don't know if four helicopters crossed the border. I don't know whose helicopters they would be, but they were not NATO or ISAF," said NATO spokesman James Appathurai.

"I don't have any more information; there has been no NATO or ISAF involvement in crossing the border into Pakistan."

The United States leads a separate coalition force in Afghanistan that focuses on counter-insurgency operations.

Asked about the raid, a Pentagon spokesman, Bryan Whitman, said: "I have nothing for you on those reports."

The CIA referred all questions to the Pentagon.

Since the emergence of a civilian-led government in Pakistan after February elections, there has been mounting concern that US military operations were becoming more aggressive in the tribal lands bordering Afghanistan.

The number of missile attacks launched by drone aircraft have multiplied, and there have been fears US forces would use helicopter gunships or put troops on the ground for ‘hot pursuit’ or commando-style raids to destroy al-Qaeda nests.

A spokesman for the Pakistani embassy in Washington said the force that conducted the raid was acting on faulty intelligence that was never shared with Pakistani forces.