After 10 years of war in Afghanistan, a persistent lack of trust between the U.S. and Pakistan still complicates operations along the critical Afghanistan border and was a key factor in the errant American airstrikes late last month that killed 24 Pakistani troops.

U.S. officials on Thursday accepted some blame for the deadly incident that infuriated Pakistani leaders, prompting Pakistan to shut down key supply routes for the war and further eroding America's already rocky relations with Islamabad. The Defense Department briefed reporters Thursday on the conclusions reached in its investigation into the November incident.

But the U.S. did not apologize, despite the embarrassing series of communications and coordination errors. Pentagon officials said Thursday that military leaders had spoken by phone to Pakistan army chief Gen. Ashfaq Pervez Kayani about the report's conclusions, but have not yet given him a detailed briefing.

Pakistan refused to cooperate in the investigation. And the U.S. report _ placing some of the blame on Islamabad _ is likely to only increase their fury, hamper any hope of rebuilding the relationship and delay the opening of the supply routes.

In a Pentagon briefing, Brig. Gen. Stephen Clark, an Air Force special operations officer who led the investigation, made it clear that U.S. forces were fired on first and acted in self-defense.

But he acknowledged that efforts to determine who was firing on the U.S troops and whether there were friendly Pakistani forces in the area _ the primary questions in any cross-border incident _ failed because U.S. forces used inaccurate maps, were unaware of Pakistani border post locations and mistakenly provided the wrong location for the troops.

There is "an overarching lack of trust between the two sides" that keeps them from giving each other specific details on troops or combat outpost locations, Clark said as he went through a blow-by-blow account of the events that began late on Nov. 25 and continued overnight.

U.S. and NATO commanders, Clark said, believe that some of their military operations have been compromised when they've given details and locations to the Pakistanis.

According to Clark, U.S. troops were climbing up rugged terrain toward a village just west of the border when they began to receive machine gun and mortar fire very close to their positions. The U.S. ground commander requested a show of force, so an F-15 fighter jet and an AC-130 gunship flew over, shooting flares to signal the presence of American or NATO troops.

Clark said the gunfire and mortars continued. And in the first serious miscommunication, the troops on the ground were told that no Pakistani troops were in the area. Commanders then called for airstrikes.

In a confusing series of communications, U.S. officials gave Pakistan liaison officers the wrong location of the firefight and were told again that no Pakistani troops were in that region. The U.S. launched another round of airstrikes until around 1 a.m., when officials confirmed that there were friendly troops there and the firing stopped.

A key failing, Clark said, was that U.S. troops did not know that two relatively new and spare Pakistani outposts _ reportedly called Volcano and Boulder _ were just over the border from the village that was the target of the operation.

"They didn't have coordinates on the border posts to begin with because they didn't know they were there," Clark said. "The border was not considered a factor to the operation because everything was intended to remain within a kilometer, kilometer and a half inside of Afghanistan. So they never anticipated taking fire from the ridgeline, nor anticipated the idea that it might be Pakistan military there." A kilometer is about 0.62 miles.

He said that as a result the U.S. troops believed enemy insurgents were firing at them. He added that U.S. commanders in Afghanistan will make any decisions on whether anyone should be punished for the mistakes.

"For the loss of life and for the lack of proper coordination between U.S. and Pakistani forces that contributed to those losses, we express our deepest regret," Pentagon spokesman George Little told reporters.

Little said the U.S wants to learn from the mistakes and take any corrective measures needed to make sure such mistakes aren't repeated.

NATO, Afghanistan and Pakistani forces use the joint border control centers to share information and coordinate security operations.

The Pakistani army rejected the U.S. investigation. The army sent a short statement to reporters early Friday in Islamabad saying media reports of the investigation indicated it was "short on facts." It said a more detailed response would be given when the report was received.

Afghan officials had no immediate comment.

The Pakistani military has said it provided NATO with maps that clearly showed where the border posts were located.

Since the attack, a furious Pakistani government has shut down NATO supply routes to Afghanistan and thrown the U.S. out of its Shamsi Air Base in southwestern Baluchistan province. The base was used to maintain drones deployed in strikes against insurgents hiding in safe havens in Pakistan's lawless tribal belt on the Afghan frontier.

The Pakistani border closure forced the U.S. and NATO to reorient their entire logistics chains to the so-called Northern Distribution Network through Russia and Central Asia.

For most of the 10-year war in Afghanistan, 90 percent of supplies shipped to the international force came through Pakistan, via the port of Karachi. But over the past three years, road and rail shipments from NATO's European members via Russia and the Central Asian nations have expanded, and before the border incident accounted for more than half of all overland deliveries.