Pakistan is refusing to participate in the U.S. investigation of last week's NATO bombing along the Afghanistan border that killed 24 Pakistani soldiers, a probe that officials say could put much of the blame on Pakistan's military.

At a Pentagon briefing Friday, Defense Department press secretary George Little said the U.S. asked Pakistan to be part of the investigation, but the Pakistanis have "elected to date" not to participate.

That decision is just one in a series of actions Islamabad has taken in retribution for the incident, including a move to shut down supply lines that the United States uses to get food, fuel and other equipment from ships at Pakistani ports on the Arabian Sea to troops in Afghanistan.

At the same time, however, U.S. military officials said Friday that Pakistan has continued to have representatives at the border coordination centers working with Afghan and coalition forces.

So far, the U.S. has sufficient supplies on hand and has been able to find other ways to get shipments through to Afghanistan, including alternate routes through Uzbekistan.

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"Logistics is about alternatives; it's about options, and we're certainly working through what sort of options we may need to pursue," said Navy Capt. John Kirby, a Pentagon spokesman.

Asked why the U.S. has chosen to express its regrets for the incident but has not explicitly apologized to the Pakistanis, Kirby said that "in this case, this was clearly a military engagement ... that cross-border fire resulted in the deaths of some two dozen Pakistani soldiers, not innocent villagers or civilians."

Kirby added that it is too early to assign blame for the deaths because the investigation is not yet completed.

Other defense officials have told The Associated Press that when U.S. and Afghan forces came under fire, the U.S. checked with the Pakistan military to see if there were friendly troops in the area and were told there were none.

Pakistani officials have said that the U.S. gave them the wrong location when asking clearance to return fire.

Defense officials deny that, and say the investigation is likely to point substantial blame at the Pakistanis.

The U.S. officials spoke on condition of anonymity because the investigation is not complete.

German Brig. Gen. Carsten Jacobson, a spokesman for the NATO coalition in Afghanistan, said it will take much of December to complete the investigation.

"We need Pakistan to join this process so that we have a rounded picture," he said, adding, however, that, "we have to understand that emotions obviously are very high on the Pakistani side."

The incident has deteriorated the already shaky U.S. relationship with Pakistan, endangering the war in Afghanistan, including reconciliation efforts with members of the Afghan Taliban who often seek sanctuary in Pakistan.

But defense officials said that so far the retribution from Pakistan, including the shutdown of supply lines, has had negligible effect on the war effort, including operations to go after insurgents.

"I don't think, if I was a member of the Haqqani network or the Taliban, that I'd be lollygagging around that area," said Kirby.

U.S. Defense and State Department officials also said the U.S. continues to reach out to Pakistan and hopes that the relationship can be mended.

At the State Department, spokesman Mark Toner told reporters that efforts were continuing to persuade Pakistan to participate in next week's conference in Bonn, Germany, on the future of Afghanistan. The Pakistanis withdrew from the event this week.

"We think it would be regrettable if Pakistan were not to attend this conference," Toner said. "It's important for the region. It's important for the neighborhood. It's important that we all work to put Afghanistan on a square and solid footing."

Toner said that the U.S. and Pakistan had a shared interest in combating extremist groups operating in Pakistani-Afghan border areas and that cooperation needed to be improved.