WASHINGTON (Reuters) - Pakistan and the United States depend on one another too much to allow the deaths of two dozen Pakistani soldiers in airstrikes by NATO forces on Saturday to cause a definitive rupture.
But the incident, the latest in a series of embarrassments this year to bedevil the relationship between two ostensible allies, will only aggravate the mistrust between the countries, and will require quick diplomatic work to contain.
Analysts and Western officials who track the relationship said a speedy, thorough investigation to find out what happened, establish responsibility and make amends is vital, although any reconciliation may be harder to achieve if NATO forces conclude the Pakistani side started the fight.
"They still have a great deal of co-dependence," said Shuja Nawaz, an authority on the Pakistani military at the Atlantic Council think tank in Washington. "The United States needs Pakistan until it wraps up kinetic operations in Afghanistan."
The United States plans to have most troops out of Afghanistan by the end of 2014.
"Pakistan, of course, is still fairly heavily dependent on U.S. financial and military support," Nawaz said. "But the way things have been going this past year, it's one event after another."
All the details of what happened in the latest incident, in Pakistan's Mohmand tribal agency, are not yet publicly known.
NATO helicopters and fighter jets based in Afghanistan attacked two Pakistan military outposts on Saturday, killing 24 Pakistani soldiers in what Islamabad called an unprovoked assault.
A Western official and a senior Afghan security official on Sunday said that NATO and Afghan forces came under fire from across the border with Pakistan before NATO aircraft attacked the Pakistani forces.
An early test of how much the U.S.-Pakistani relationship has been hurt may come from how well the sides cooperate with one another and with the Afghan authorities to establish precisely what happened on the border.
The key questions include who fired first and from where; why NATO and Pakistani forces appear to have been unable to communicate so as to prevent the Pakistani deaths; and whether NATO helicopters knew they had entered Pakistani territory.
"All of this is extremely murky and needs to be investigated," said an Obama administration official who spoke on condition of anonymity.
"Our goal today is ... that the investigation gets mounted in a way that is confidence-building on all sides," the official added.
Pakistan shut down NATO supply routes into Afghanistan -- used to send in nearly half of the alliance's land shipments -- in retaliation for the incident, the worst such attack since Islamabad uneasily allied itself with Washington following the September 11, 2001, attacks on the United States.
Islamabad also said it would ask U.S. personnel to vacate a Pakistani base used to launch drone attacks, a threat it has made before without following through.
The NATO attack was the latest perceived provocation by the United States, which infuriated Pakistan's powerful military with a unilateral U.S. special forces raid that killed al Qaeda leader Osama bin Laden in May.
That raid also cost Pakistan much goodwill among U.S. politicians who questioned why the United States provides so much military and economic assistance to a country where bin Laden lived with impunity. Many Republican presidential candidates have asked the same question.
According to a U.S. Congressional Research Service tally released this year, the United States set aside some $22.01 billion in aid for Pakistan over the last decade, of which $14.62 billion was security-related and the rest economic.
Other alliance-straining events over the past 15 months included a September 30, 2010 incident in which NATO forces killed two Pakistani service personnel, leading Pakistan to cut off NATO's vital ground supply route for 10 days.
On January 27, a CIA contractor killed two Pakistani men he said were trying to rob him in Lahore, undermining ties between the U.S. and Pakistani intelligence services.
An in September, the then top U.S. military officer accused Pakistani intelligence of backing violence against U.S. targets including a September 13 attack on the U.S. embassy in Kabul.
Chairman of the U.S. Joint Chiefs of Staff Admiral Michael Mullen said Pakistan's Inter-Services Intelligence agency (ISI) had supported militants known as the Haqqani network, which he described as a "veritable arm" of the ISI.
"Is this the last straw? (I) hope not. I believe both governments also hope not," said retired Ambassador Teresita Schaffer, a fellow at the Brookings Institution who served as a U.S. diplomat in Pakistan, India, Bangladesh and Sri Lanka.
While there have been suggestions Pakistan could seek to improve its ties to China as a strategic counterweight to the United States, analysts dismissed this idea.
Islamabad receives significant amounts of military hardware from Beijing and their armed forces are close but Schaffer said the United States is a source of two things Beijing does not provide: top-flight weaponry and extensive cash assistance.
Even if there is no radical rupture, relations are unlikely to improve quickly.
"The U.S.-Pakistan relationship appears destined to lurch from crisis to crisis unless and until the two sides can reach some kind of understanding on the way forward in Afghanistan," said Lisa Curtis of the Heritage Foundation think tank.
With NATO planning to intensify its operations in eastern Afghanistan next year to try to cut off insurgent routes from Pakistan, Curtis said "the situation is likely to get worse before it gets better."
(Editing by Bill Trott)