* Fourth border raid in recent days
* ISAF says helicopters did not cross into Pakistan
* Matter under investigation
(Updates with ISAF comments)
By Javed Hussain
PARACHINAR, Pakistan, Sept 30 (Reuters) - NATO helicopters from Afghanistan attacked a militant-infested border region of Pakistan on Thursday, killing three Pakistani soldiers, a Pakistani official said, a raid that is certain to raise tensions.
A spokeswoman for the NATO-led International Security Assistance Force in Kabul, however, said none of its helicopters had crossed into Pakistani airspace.
Pakistan has said it would consider "response options" if NATO forces continued to violate its sovereignty.
On Thursaday, two NATO helicopters attacked Teri Mangal village in Kurram, an ethnic Pashtun tribal region in the Pakistani northwest, a Pakistani security official said.
"The helicopters shelled the area for about 25 minutes. Three of our soldiers manning a border post were killed and three wounded," the official, who spoke on condition of anonymity, said.
Thursday&${esc.hash}39;s attack if confirmed, would be the fourth cross-border raid in recent days, which comes just as the United States steps up strikes by unmanned drone aircraft in Pakistan&${esc.hash}39;s North Waziristan.
ISAF spokeswoman Major Sunset Belinsky said the helicopters targeted militants in Afghanistan&${esc.hash}39;s eastern Paktia province, opposite Kurram, and they did not cross into Pakistan.
But Pakistan military officials had informed ISAF that their border forces had been struck in the attack, she said in a statement.
"ISAF is working with Pakistan to ascertain if the two events are linked. The matter remains under investigation," she said.
NATO SUPPLIES
A Pakistani security official said authorities had stopped trucks carrying supplies for the NATO forces in Afghanistan at a checkpost in neighbouring Khyber region after the incident.
"Yes, the NATO supplies have been stopped. It has been done locally," he told Reuters.
The rugged border region between Afghanistan and Pakistan is seen by Washington as a critical battleground in its fight against al Qaeda and the Taliban.
Though many analysts believe that the strikes by unmanned U.S. drones are carried out with the tacit approval of Pakistan, any border incursions by foreign troops is a highly explosive issue in Pakistan where anti-American sentiments run very high.
In 2008, Pakistani troops had fired on US military helicopters and forced them to return to Afghanistan after Pakistan army chief General Ashfaque Kayani said Pakistan would not allow foreign troops on its soil.
The latest series of raids began last Friday when two NATO Apache helicopters killed 30 insurgents on Pakistani soil after a rare manned pursuit across the border from eastern Afghanistan. It followed an attack by militants on a remote Afghan security outpost in Khost province, NATO said.
On Saturday, two Kiowa helicopters returned to the area and killed another four. Monday saw another possible border violation, with six militants killed in Kurram, a Reuters reporter in the area said. But an ISAF spokesman said it was "near the border," rather than in Pakistan.
ISAF said in a statement issued late on Sunday that helicopters crossing into Pakistan were following its rules of engagement. (Additional reporting by Zeeshan Haider and Emma Graham-Harrison; Writing by Zeeshan Haider; Editing by Chris Allbritton and Sanjeev Miglani) (For more Reuters coverage of Afghanistan and Pakistan, see: http://www.reuters.com/places/pakistan.
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