Firing by Pakistani troops forced US military helicopters to turn back to Afghanistan after they crossed into Pakistani territory early on Monday, Pakistani security officials said.

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. -Reuters