WASHINGTON — U.S. and NATO forces have killed more than 100 fighters from a Pakistan-based faction of the Taliban during two weeks of stepped-up military operations along the Afghanistan-Pakistan border, the Pentagon said Tuesday.

The intensified border operations have contributed to tension between the United States and Pakistan that reached a critical point last week after U.S. forces crossed into Pakistan and mistakenly killed three Pakistani frontier soldiers. Pakistan closed a major border crossing used to supply fuel to U.S. forces in Afghanistan in retaliation.

Pentagon press secretary Geoff Morrell expressed remorse for what he called a mistake by U.S. forces and said results of a joint NATO-Afghan investigation of the incident would be released Wednesday.

Morrell said U.S. and NATO forces targeted Haqqani network fighters using border areas as a refuge in eastern Afghanistan. He said more than 100 were killed, and later gave a more precise figure.

The tally was an example of an increasing U.S. willingness to provide figures for enemy deaths in a counterinsurgency fight that U.S. commanders long have insisted can never be won by attrition.

"The threat is real, and though we've had success in killing 110 of them, there clearly are more of them out there who remain a threat to our forces," Morrell said, adding that the radical Islamic fighters also battle Afghan and Pakistani forces.

The Haqqani network is a Pakistan-based faction of the Taliban with close ties to al-Qaida.

The group was started by Jalaluddin Haqqani, a commander supported by Pakistan and the United States during the 1980s war against the Soviet Union in Afghanistan. Haqqani has since turned against the United States, and American military officials have said his organization, now effectively led by his son, Sirajuddin, presents one of the greatest threats to foreign forces in Afghanistan.

Morrell said he has not heard anything to suggest the United States will change the way its aircraft operate along the border, although he would not discuss specific rules of engagement.

"We will retain the right to defend our forces, to defend ourselves," he said during a Pentagon news conference. "And our forces who operate on the border with Pakistan are in a very dangerous and difficult situation."

The helicopter strike that killed the three Frontier Corps men was the third such incursion into Pakistan in less than a week.

Pakistan is extremely sensitive to any U.S. military presence inside its borders, and conspiracy theories about U.S. motives abound. The incidents further opened Pakistan's weak civilian government to charges at home that it is a U.S. puppet.

The stepped-up operations coincided with but are not directly tied to the three cross-border incursions, which U.S. officials acknowledge puts additional pressure on Islamabad to respond.

"There are occasional setbacks in our day-to-day relations, this being one of them, the most recent of them," Morrell said.

Other U.S. officials stressed that the United States is not intentionally squeezing Pakistan to make the point that if it fails to target militants vigorously the United States will step in.

A small bomb damaged a truck in northwestern Pakistan on Tuesday that was carrying oil to NATO troops in Afghanistan, the latest attack on stalled supply convoys since Pakistan shut the border crossing last week.

U.S. and Pakistani officials have said the Torkham crossing probably would be reopened within a few days. U.S. military officials said the closure has not harmed delivery of fuel to U.S. forces, although alternate routes are less convenient and more expensive.

The increased military operations along the border and CIA drone strikes farther inland are an outgrowth of better intelligence and targeting information flowing from sources inside and outside Pakistan, officials said. At the same time they represent a calculated risk that the tense partnership between Washington and Islamabad can withstand the inevitable backlash.

"There are mistakes. There are incidents which create misunderstandings," Morrell said.

"But that does not mean the relationship, this crucial relationship to us, is in any way derailed."

Besides safe passage for NATO supplies, the United States needs Pakistan to help target the Taliban and al-Qaida militants who stage cross-border attacks on troops in Afghanistan. Pakistan meanwhile receives billions of dollars in military and civilian aid that help keep its economy afloat. (AP)