ISLAMABAD (Reuters) -- Fierce firing by Pakistani troops forced U.S. 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 U.S. commandos in helicopters raided earlier this month.
“The U.S. choppers came into Pakistan by just 100 to 150 meters at Angor Adda. Even then our troops did not spare them, opened fire on them and they turned away,” said one security official.
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 U.S. armored vehicles were also seen moving on the Afghan side of the border, while U.S. 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 U.S. 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 U.S. military at Bagram Airbase, north of Kabul, said its forces had not reported any such incident.
“The unit in the area belongs to the (U.S.-led) coalition. They are not reporting any such incident,” the U.S. 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 U.S. President George W. Bush has given clearance for U.S. raids across the border.
The raid on Angor Adda on September 3 was the first overt ground incursion by U.S. troops into Pakistan since the deployment of U.S. forces in Afghanistan in late 2001.
The United States has intensified attacks by missile-firing drone aircraft in Pakistani tribal lands in the past few weeks.