The incident took place near Angor Adda, a village in the tribal region of South Waziristan where US commandos in helicopters raided a suspected al Qaeda and Taliban camp earlier this month.

The US choppers came into Pakistan by just 100 to 150 metres at Angor Adda. Even then our troops did not spare them, opened fire on them and they turned away, said one security official.

The US and Pakistani military both denied that account, but Angor Adda villagers and officials supported it.

Pakistan is a crucial US ally in its war on terrorism, and its support is key to the success of Western forces trying to stabilise Afghanistan. But Washington has become impatient over Islamabad’s response to the threat from al Qaeda and Taliban fighters in Pakistan’s tribal regions on the border.

At least 20 people, including women and children, were killed in the South Waziristan raid earlier this month, sparking outrage in Pakistan and prompting a diplomatic protest.

Pakistani army chief General Ashfaq Kayani said in a strongly worded statement last week that Pakistan would not allow foreign troops onto its soil and Pakistan’s sovereignty and territorial integrity would be defended at all costs.

Another security official said on Monday that US armoured vehicles were also seen moving on the Afghan side of the border, while US warplanes were seen overhead.

He said Pakistani soldiers sounded a bugle call and fired in the air, forcing the helicopters to return to Afghan territory.

Military spokesman Major Murad Khan confirmed that there had been shooting. But he said the American helicopters had not crossed into Pakistani airspace and Pakistani troops were not responsible for the firing.

The US choppers were there at the border, but they did not violate our airspace, Khan said.

We confirm that there was a firing incident at the time when the helicopters were there, but our forces were not involved.

A spokesman for the US military at Bagram Airbase, north of Kabul, said its forces had not reported any such incident.

The unit in the area belongs to the (US-led) coalition. They are not reporting any such incident, the US military spokesman said.

But the official denials were contradicted by Pakistani civilian officials and villagers in Angor Adda.

One official told Reuters by telephone that the troops stationed at BP-27 post fired at the choppers and they turned away.

Two Chinook helicopters appeared set to land when troops began shooting, alerting tribesmen who also opened fire on the intruders, said a senior government official in Peshawar, the capital of North West Frontier Province.

A resident described the tension in the village through the night. We saw helicopters flying all over the area. We stayed awake the whole night after the incident, he said.

The fiercely independent tribesmen of the region carry weapons regardless of whether they are militants.

The New York Times newspaper reported last week that US President George W. Bush has given clearance for US raids across the border.

The raid on Angor Adda on September 3 was the first overt ground incursion by US troops into Pakistan since the deployment of US forces in Afghanistan in late 2001.

The United States has intensified attacks by missile-firing drone aircraft on suspected al Qaeda and Taliban targets in Pakistani tribal lands in the past few weeks.

Despite apparent US frustration with Pakistan, the Pakistani army has been involved in fierce fighting with Islamist militants in Bajaur, another tribal region, and Swat, a valley in North West Frontier Province, close to the tribal lands.

Pakistani forces, using helicopter gunships and artillery, killed at least 16 fighters and wounded 25 in Bajaur on Sunday. More than 750 militants have been killed in an offensive there that began in late August.