Pakistan pulled out of an upcoming meeting in Germany on the future of Afghanistan to protest the deadly attack by U.S.-led forces on its troops, widening the fallout on Tuesday from an incident that has sent ties between Washington and Islamabad into a tailspin.

Meanwhile, a top Pakistani army general called the incident Saturday that killed 24 troops on the Afghan-Pakistan border a "deliberate act of aggression" by NATO forces and said the military had not decided whether to take part in an American investigation into it.

Both developments bode ill for future Pakistani cooperation with U.S. and NATO forces in Afghanistan, and reflect the hardline being taken by the army, which is under pressure by an anti-American public to respond forcefully. NATO has described the incident as "tragic and unintended", and U.S. officials have expressed their sympathies with the families of those who died.

The decision to skip the conference in Bonn, Germany, which has been a year in the planning, will trigger concerns in Washington and Kabul that Pakistan is withdrawing from international efforts to stabilize Afghanistan before and after the withdrawal of foreign combat forces in 2014.

It was taken during a Pakistani Cabinet meeting in the city of Lahore, said three officials who attended the meeting. They spoke on condition of anonymity because they were not authorized to talk to the media ahead of an official announcement.

German Chancellor Angela Merkel said she understood Pakistan's reason for not attending, but that she hoped it would reconsider its decision. "They should still understand that the Afghanistan conference is a very important one. It's a very good opportunity to bring forward the political process," she said.

Pakistan, which has long had a troubled relationship with Washington, has already closed its two crossings on the western border to trucks delivering supplies to NATO troops in landlocked Afghanistan and said it will review all cooperation with NATO and the United States.

The Dec. 5 Bonn meeting was to bring together Western and regional leaders to forge a strategy to stabilize Afghanistan and smooth the planned American withdrawal from the country in 2014. Pakistan is perhaps the most important regional country because of it influence on Afghan Taliban factions on its soil, and U.S. and Pakistani officials had been urging Islamabad to attend.

Given the general pessimism about the future of Afghanistan, few had high expectations the conference would result in significant progress. But the absence of Pakistan will make even minor achievements much more difficult.

There have been conflicting versions of what led to the attack by NATO aircraft on Saturday, though most Afghan and Western accounts say it was likely a case of friendly fire, launched after a joint Afghan and U.S. special forces team received fire from the Pakistan side of the border.

But Pakistan army Gen. Ashfaq Nadeem on Tuesday called the incident a "deliberate act of aggression" and said it was "next to impossible that NATO" did not know they were attacking Pakistani forces. Nadeem made the remarks to a briefing of Pakistani news anchors, senior journalists and defense analysts at army headquarters.

Foreign media were not invited, but two attendees relayed to The Associated Press what Nadeem said.

One was analyst and retired Gen. Talat Masood; the other didn't give his name because he feared his employers might not approve.

Nadeem said the army had little faith that any investigation will get to the bottom of the incident and may not cooperate with it. He said other joint inquiries into at least two other similar, if less deadly, incidents over the last three years had "come to nothing."

A NATO official, speaking on condition of anonymity to discuss an incident still being investigated, said between 50 and 150 Afghan commandoes and U.S. Special Forces participated in what the official described as a "normal operation." The joint force was targeting Afghan insurgents in Kunar province, not a high-value individual, such as Taliban leader Mullah Omar, the official said.

Pakistan is angry over the death of its soldiers, but its leaders appear to be playing up their outrage, in part to seek fresh leverage in Islamabad's relationship with Washington. Despite the mistrust, neither side wants to break ties entirely.

Anthony Cordesman, an analyst for the Center for Strategic and International Studies in Washington, said it's likely that this crisis "will get papered over" with some sort of U.S. or NATO apology and a "bribe in the form of better aid flows."

"In the process, however, the U.S. will face even less prospect that Pakistan will really crack down on insurgent groups in the border area, or stop seeing Afghanistan as an area where it competes with India and which is useful for strategic depth in some future war with India," said Cordesman.

Last year, Pakistan closed one of the border crossings for 10 days after a U.S. chopper killed two Pakistani soldiers on the border in a friendly fire incident. Militants then attacked dozens of the stranded supply trucks that were lined up by the side of the road. After 10 days, the U.S. apologized and it reopened the border.

German Brig. Gen. Carsten Jacobson, a coalition spokesman in Afghanistan, told reporters at a regular briefing in Kabul said the NATO coalition has stopped dispatching convoys from Karachi, the sea port in southern Pakistan where the supplies are unloaded, to stop trucks massing at the two closed border crossings.

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Associated Press writers Zarar Khan and Asif Shahzad in Islamabad, and Deb Reichmann in Kabul, contributed to this report.