ISLAMABAD, Sept 15 - Firing by Pakistani troops forced U.S. military helicopters to turn back to Afghanistan after they crossed into Pakistani territory in the early hours of 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 had carried out a heli-borne raid earlier this month. At least 20 people, including women and children, were killed, 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 cost.