US Secretary of State Hillary Clinton led calls on Pakistan on Wednesday to reconsider boycotting talks on Afghanistan, but stopped short of apologising for the deaths of 24 soldiers in NATO strikes.
The Pakistani cabinet took the decision in protest against Saturday's attack high in the mountains on the Afghan-Pakistani border, the worst ever cross-border attack by US-led NATO troops in 10 years of war in Afghanistan.
Pakistan has closed the Afghan border to NATO convoys, a lifeline for 140,000 foreign troops in Afghanistan, ordered American personnel to vacate an air base reportedly used by CIA drones and launched a review of the alliance.
Clinton voiced regret over Pakistan's decision, pledging an investigation "as swiftly and thoroughly as possible" into the "tragic incident" and hoping it would find a "follow-up way" to take part in talks on Afghanistan's future.
German Chancellor Angela Merkel said Berlin would "see what could be done to change" Islamabad's decision. Afghan President Hamid Karzai also telephoned Prime Minister Yousuf Raza Gilani, asking Pakistan to re-think its attendance.
As a neighbour with historic ties to the Taliban, Pakistan is considered integral to ending the decade-long conflict, but experts say a boycott matters less now that expectations for Bonn have been dramatically curtailed.
Diplomats had hoped the conference would help ignite peace with the Taliban, who will not be attending either.
But the September assassination of Kabul peace envoy, Burhanuddin Rabbani, by a reputed Taliban emissary dealt a heavy blow, forcing Western and Afghan officials to concede that contacts with insurgents have achieved little.
One diplomatic official in Afghanistan called on the Americans to "act expeditiously" to stave off a boycott.
"If they made a mistake, they have not only to apologise but they have to do something more than that, compensation or something else because that's pretty serious," the official added.
A Western diplomat said Pakistan, viewed by many internationally as a black sheep responsible for violence in Afghanistan, could use the boycott to whip up support "because everyone will appeal on them to come to Bonn".
On the agenda in Bonn is the ongoing process of transition from NATO to Afghan control, long-term international engagement and hopes for integration and reconciliation, said EU envoy to Kabul, Vygaudas Usackas.
"It won't be about reconciliation it will be about long-term commitment, credible, mutual long-term commitment from both sides," Usackas said, adding that reconciliation would have to come from within Afghan society.
"I think something could still be done even without Pakistan because it was never going to be focused only on the Taliban and Pakistan," said Fabrizio Foschini from the Afghanistan Analysts Network.
US-Pakistan ties have been in free fall since a CIA contractor killed two Pakistanis in Lahore in January. But both countries are mutually dependent.
Islamabad relies on US aid, while the Americans depend on Pakistani logistical support and want them to do more to fight Afghan Taliban.
Islamabad insists that the air strike was unprovoked, but Afghan and Western officials have reportedly accused Pakistani forces of firing first.
On Tuesday, Major General Ishfaq Nadeem briefed a hand-picked group of local journalists, reiterating that the air strikes were unprovoked and saying they continued despite Pakistani protestations to the Americans.
"The positions of the posts were already conveyed to the ISAF (NATO force) through map references and it was impossible that they did not know these to be our posts," he was quoted as saying by state media.
Pakistan has cleared the area of militants and there was no cross border movement of insurgents from Pakistan to Afghan territory, he added.
After midnight on November 26, two to three helicopters appeared and opened fire on a Pakistani border post. A second post retaliated by firing anti-aircraft guns and all available weapons, and was then attacked, he said.
"We informed them about the attack. But, the helicopters reappeared and also engaged the Boulder post," Nadeem was quoted as saying.
The US military has given investigators until December 23 to submit initial findings on what happened exactly.
Pakistani Cable television operators Tuesday blocked the BBC's world news channel in protest at a documentary they said was "anti-Pakistan" and threatened to suspend other Western broadcasters.