* Three Pakistani soldiers killed in attack
* ISAF: Aircraft entered Pakistan, killed "armed individuals"
* Pakistani PM expresses concerns on border incursions (Adds details about NATO supply routes, paragraphs 12-13)
By Zeeshan Haider
ISLAMABAD, Sept 30 (Reuters) - Pakistani authorities blocked a vital supply route for NATO troops fighting in Afghanistan on Thursday after the killing of three soldiers in two NATO cross-border incursions, officials said.
Trucks and fuel tankers for foreign forces in Afghanistan were stopped at the Torkham border post in the Khyber tribal region near the city of Peshawar, hours after the raid.
"Yes, the NATO supplies have been stopped. It has been done locally," a senior security official told Reuters on condition of anonymity.
Aircraft from the NATO-led International Security Assistance Force (ISAF) initially crossed the border in the Kurram region briefly while targeting suspected insurgents who were firing on a coalition base from a position inside Afghanistan, an ISAF statement said.
They were then fired on by people in Pakistan, and crossed the border again to target that group.
"Operating in self-defense, the ISAF aircraft entered into Pakistani airspace, killing several armed individuals," the statement said.
The statement did not say if ISAF thought those killed were border guards and when asked for clarification, an ISAF spokeswoman said both sides still were investigating the incident.
"This is the third incident of its kind during the past week," the Pakistani military said in a statement. Three soldiers were wounded, it said.
Pentagon spokesman Colonel David Lapan declined to elaborate on the details of the incident but said a Pakistani statement said "their forces used rifle fire at the helicopters as a warning."
"You fire at helicopter in a combat zone, they usually take that as hostile and return fire," he said.
Pakistan is a crucial ally for the United States in its efforts to stabilize Afghanistan, but analysts say border incursions and disruptions in NATO supplies underline growing tensions in the relationship.
About half of all cargo for NATO forces in Afghanistan travels through Pakistan, most of it via two main border crossings: Chaman and Torkham, the Pentagon said.
Another third flows into Afghanistan through the northern distribution network across Kazakhstan and Uzbekistan. Sensitive gear like ammunition, weapons and critical equipment is flown in, the Pentagon said.
Lapan said the Torkham border crossing closure had had "no immediate impact" on NATO resupply but its possible effect would depend on how long it remained shut.
He said NATO has a variety of supply routes into Afghanistan, including others in Pakistan that remained open.
"We are in discussions with the Pakistani government and hope that we can resolve the issue," Lapan said.
The border row occurred as CIA chief Leon Panetta began a previously scheduled visit to Pakistan for talks with top military and political figures.
Panetta met President Asif Ali Zardari, Prime Minister Yusuf Raza Gilani, army chief General Ashfaq Kayani and the head of the powerful Inter-Services Intelligence (ISI) agency, Lieutenant General Ahmed Shuja Pasha.
Gilani expressed "profound concern" over increasing drone strikes and violations of Pakistan's airspace by NATO forces, he said in a statement.
Citing reports by Western security officials about foiling a militant plot to stage coordinated attacks in Europe, Gilani pledged Pakistan's help to thwart such plans if given "credible information in advance."
Panetta welcomed Pakistan's help in the fight against militants and said the U.S. government would look into its ally's complaints about border violations.
A senior intelligence official said border incursions into Pakistan were a "red line" and could lead to a "total snapping of relations."
Neither country could afford that, the official said, so it would be a disaster if further incursions took place.
"But we'll live with that or we'll die with that," he said. "We're in a state of war. We've lost more than 30,000 people since 2001. What more can we lose? Another 100,000? These incursions are not something we can tolerate."
Pakistan has said it would consider "response options" if NATO forces continued to violate its sovereignty.
Washington has stepped up missile strikes by unmanned drone planes in Pakistan's northwest, carrying out 21 in September, the highest for a month since it began such attacks in 2008.
Also Thursday, a video purporting to show Pakistani troops in the northwestern Swat region summarily executing a group of bound and blindfolded young men appeared on the Internet.
The military is investigating, the intelligence official said, although he believed it was likely a forgery by the Pakistani Taliban, distributed as propaganda. (Additional reporting by Chris Allbritton, Javed Hussain and Kamran Haider; Editing by Ron Popeski, Philip Barbara and Bill Trott (For more Reuters coverage of Afghanistan and Pakistan, see: http://www.reuters.com/places/pakistan.)
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