By Mark Hosenball
WASHINGTON (Reuters) - Militants based in Pakistani safe havens are stepping up cross-border attacks in Afghanistan as security cooperation between the United States and Pakistan remains under severe strain, U.S. officials said.
On Tuesday, 16 people were reported killed after armed militants laid siege for hours to the American Embassy and NATO headquarters in Kabul, the Afghan capital.
The U.S. ambassador to Afghanistan, Ryan Crocker, attributed the attack to the Haqqani network, a group allied with al Qaeda and the Taliban based on the Pakistani side of the Afghan-Pakistan border.
U.S. officials said they believed the Haqqani network also carried out a truck bombing on Saturday at a NATO outpost in Afghanistan's Wardak province that killed four civilians and wounded 77 U.S. troops.
Defense Secretary Leon Panetta warned Pakistan on Wednesday the United States would "do everything we can" to defend U.S. forces from Pakistan-based militants staging attacks in Afghanistan.
"Time and again we've urged the Pakistanis to exercise their influence over these kinds of attacks from the Haqqanis. And we have made very little progress in that area," Panetta told reporters.
"I think the message they (the Pakistanis) need to know is: we're going to do everything we can to defend our forces," he added.
Several U.S. government officials said efforts by the United States and NATO to wipe out, or at least curb, militants in Afghanistan were unlikely to succeed as long as groups like the Haqqani network can operate with relative impunity from Pakistan's remote tribal areas.
But one U.S. official noted that just because small groups of Haqqani militants could mount attacks against high-profile targets in Kabul, a long way from their safe haven, did not mean the militants had free rein around the country.
"The Haqqanis are a significant threat to security in Afghanistan, but they're not exactly in a position to take over the country," the official said.
The network's suspected hide-outs in Pakistan have become major targets for CIA drone strikes. But U.S. counterterrorism officials said Pakistani officials had been pressuring the United States for nearly a year to curb its clandestine operations inside Pakistan.
The recent arrest, in a Pakistan-U.S. operation, of an al Qaeda leader known as Younis al Mauretani, marked a respite from tensions between the two countries. But one U.S. national security official said al Mauretani's arrest was the only positive recent development in an otherwise bleak counterterrorism relationship.
"The bilateral relationship is still in deep trouble but the atmospherics are a bit better," said Bruce Riedel, a former CIA analyst who has advised the Obama administration on policy in the region. "Name calling has largely ended for now."
There has been "some cooperation on interrogation of al Mauretani. But distrust has not gone away, nor has the fundamental difference in approach to terror," Riedel said.
A current U.S. official was more upbeat, saying, "Clearly the relationship is complicated, yet both sides still find ways to get things done together against a common enemy -- al Qaeda and their extremist allies."
Relations between the CIA and Pakistan's principal spy agency, the Inter Services Intelligence directorate, have been in a slump for months.
First, the CIA station chief in Islamabad had to leave Pakistan after his name was leaked to local media late last year. Ties deteriorated further when CIA contractor Raymond Davis was detained by Pakistani authorities after he was charged with killing two men who he said tried to rob him. Davis was later released.
Then in May, a U.S. commando team killed al Qaeda chief Osama bin Laden at a Pakistani hide-out without notifying local authorities in advance.
In the wake of those incidents, Pakistani authorities expelled at least 100 U.S. military personnel who had been training Pakistani counterterrorism forces. Pakistani authorities also began refusing new entry visas for U.S. personnel, including CIA officers.
Some American operatives already in Pakistan have been asked to extend their tours of duty so operations are not cut back drastically, officials said, but that risks straining their morale and endurance.
(Additional reporting by David Alexander; Editing by Vicki Allen and Peter Cooney)