WASHINGTON, (Reuters) – A U.S. probe on Thursday found both American and Pakistani forces were to blame for a border incident that killed 24 Pakistani troops last month, inflaming already strained ties and deepening doubts about whether Pakistan will help or hinder the U.S. fight in Afghanistan.

The U.S. military blamed Pakistani soldiers for firing at NATO forces as they prepared for a mission in a remote area near Afghanistan’s eastern border with Pakistan close to midnight on November 25.

The U.S. investigation also conceded a critical error by U.S. troops, who told Pakistan the cross-border shooting was taking place about 9 miles (14 km) away due to mapping error. Pakistan responded by saying it had no troops there.

“Inadequate coordination by U.S. and Pakistani military officers operating … resulted in a misunderstanding about the true location of Pakistani military units,” Pentagon spokesman George Little told reporters on the incident, which stretched into the early hours of November 26.

“This, coupled with other gaps in information about the activities and placement of units from both sides, contributed to the tragic result,” he said.

Pakistan rejected the probe’s findings just hours after they were made public. Army spokesman Major General Athar Abbas said Pakistan did not agree with the U.S. findings because they were “short on facts.”

The death of the Pakistani soldiers dug in along the mountainous, isolated border area, along with the initial NATO response, has incensed Pakistanis and marked yet another setback in the Obama administration’s efforts to improve chronically troubled ties with an uneasy ally.

The incident prompted Pakistan to shut down ground routes used to supply U.S. forces in Afghanistan and to demand that the United States vacate an air base used to launch drone flights within 15 days. Suggesting the strike may have been intentional, Pakistan has waived aside U.S. expressions of regret and angrily waited for a formal U.S. apology.

Pakistan’s rejection of the probe – it also rebuffed an invitation to take part – is another bad sign as the Obama administration seeks Pakistani assistance in its bid to crack down on militants who criss-cross the Pakistani border and undermine efforts to stabilize Afghanistan.

The Obama administration faces extra pressure to show results in Afghanistan as foreign forces prepare to withdraw most of their combat soldiers by the end of 2014.

SHOW OF FORCE

Brigadier General Stephen Clark, who headed the U.S. investigation, said NATO forces came under fire as they took position near an Afghan village close to the Pakistani border.

After the NATO forces began receiving mortar and machine gun fire from a ridge area, U.S. officials and a Pakistani border liaison officer embedded with NATO forces concluded – acting on an erroneous analysis of where the firefight was taking place – that no Pakistani military involved.

NATO deployed a F-15 strike fighter and AC-130 gunship which fired off flares, in what officials called “a show of force” meant to identify the troops taking fire as NATO forces. But the Pakistani troops kept firing.

Clark said that part of the problem was instructions to U.S. soldiers not to directly share details of their geographic assessments with their Pakistani liaison officers – a symptom of what he called “an overarching lack of trust” on both sides.

When U.S. forces asked Pakistan for the precise location of its forces, the exchange further illustrated mutual wariness, Clark said.

“The (Pakistani) general answer back was, ‘Well, you know where it is because you’re shooting at them,’ rather than giving a position.”

TRUST DEFICIT

In a statement released on Thursday, NATO said that a “combined international and Afghan force was initially fired upon by unidentified forces, then believed not to be Pakistani military, and legitimately responded in self-defence.”

U.S. officials said they would offer to brief the Pakistani government on the findings of the investigation and were ready to pay compensation to victims’ families.

U.S. officials said General Martin Dempsey, chairman of the U.S. Joint Chiefs of Staff, had a “professional and cordial” conversation with General Ashfaq Kayani, the head of the Pakistani military, about the report on Wednesday night.

The friendly fire incident is only the latest of a series of bilateral crises in the past year, including the U.S. raid that killed al Qaeda leader Osama bin Laden on Pakistani soil in May and the arrest in Pakistan of a CIA contractor.

It is unlikely the report will placate Pakistani leadership in the charged climate following the incident.

“To my mind this is getting very messy,” Shuja Nawaz, a scholar at the Atlantic Council in Washington.

“It is going to feed anti-American rhetoric among the (Pakistani) public and will force the Pakistani military to take a tougher position than it might have,” he said.

Yet U.S. officials said they would focus on improving cross-border communication and avoiding a repeat of such bloodshed.

“We cannot operate effectively on the border, or in other parts of our relationship, without addressing the fundamental trust still lacking between us,” Little said.

“We earnestly hope the Pakistani military will join us in bridging that gap.”