The US military was accused today of having sent a force of commandos across the border from Afghanistan into Pakistan on a raid in which 20 people were reportedly killed, including women and children.

Both Nato and the separate US-led coalition in Afghanistan denied any knowledge of the pre-dawn attack, which local residents said involved both American and Afghan troops backed by helicopter gunships.

But it was immediately seen as both undermining sovereignty and presenting a challenge to the coalition govermnet led by Yousuf Raza Gillani, the object of an unsuccessful assassination in Rawalpindi this morning.

"It is outrageous," Owais Ahmed Ghani, the governor of North West Frontier province, said in a statement on the incursion. "This is a direct assault on the sovereignty of Pakistan and the people of Pakistan expect that the armed forces of Pakistan would rise to defend the sovereignty of the country and give a befitting reply."

Habib Khan Wazir, a local resident said that the incident involved both American and Afghan troops, who attacked houses in a village called Musa Nikow. He said that he heard the sound of helicopters, and then an exchange of fire between the assailants and other residents.

"Later, I saw 15 bodies inside and outside two homes. They had been shot in the head," Mr Wazir told the Associated Press. He said the dead included women and children and that all were civilians.

"There was darkness at the time when the Americans came and killed our innocent people," he said. "We would have not allowed them to go back alive if they had come to our village in daylight."

The United States says that al-Qaeda and Taleban 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.

The attack took place near Angor Adda in the South Waziristan region, a known sanctuary for al-Qaeda and Taleban militants. There were differing accounts of the attack with some people saying it was carried out by helicopter gunships while others spoke of an attack by ground troops as well.

"Troops came in helicopters and carried out action in three houses," said Gul Nawaz, a shopkeeper in the village.

Other residents said the foreign troops detained some people and took them away. US-operated pilotless drone aircraft have launched attacks in Pakistani border regions several times this year, killing dozens of militants, but US ground troops had not been known to cross into Pakistan to fight militants.

Ahmed Mukhtar, the Pakistani Defence Minister, said that aircraft belonging to foreign forces in Afghanistan attacked three houses at 4.30am. He declined to comment further, saying the Foreign Ministry was investigating.

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