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Even as it faces the most explicit U.S. accusations yet that it supports Afghan insurgents, Pakistan is unlikely to act promptly on American demands that it move against their sanctuaries, straining an anti-terror alliance that is looking weaker than ever.

U.S. officials maintain that Pakistani inaction against militants on its territory is making it impossible for the 130,000 American troops in neighboring Afghanistan to break the insurgency before their scheduled withdrawal by the end of 2014.

The main U.S. focus is the Haqqani network, a group of Islamist militants who fight in Afghanistan and are based in the Pakistani border region of North Waziristan. American officials say the group, which is tied to Al Qaeda, is the most dangerous threat to American troops in Afghanistan.

On Thursday, Mike Mullen, the soon-to-retire chairman of the Joint Chiefs of Staff, accused Pakistan's Inter-Services Intelligence Agency of directly aiding the Haqqanis in last week's 22-hour assault against the U.S. Embassy in Kabul, one of the most serious accusations yet against a country that receives billions in American aid each year.

He didn't speculate on why the ISI would want to help the Haqqanis strike such a high-profile target, but one possibility is that the agency believes such attacks will help ensure that the Pakistanis are listened to in negotiations to end the Afghan war.

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Pakistani Foreign Minister Hina Rabbani Khar dismissed Mullen's claims as mere allegations, and warned the U.S. that it risked losing Pakistan as an ally.

"If they are choosing to do so, it will be at their own cost," Khar told Geo TV on from New York City, where she is attending a U.N. General Assembly meeting. "Anything which is said about an ally, about a partner publicly to recriminate it, to humiliate it is not acceptable."

While most Pakistani officials don't deny that the Haqqani network is present in the border region, they say America is exaggerating its reach and potency. They have suggested that Washington's focus on the network, which has roots in the CIA-backed campaign against Soviet-rule in Afghanistan in the 1980s, is meant to cover up America's failings in Afghanistan.

Pakistan has resisted U.S. calls to move against the Haqqanis, and there is little to suggest a change is in the offing.

Washington, Kabul and Islamabad all say that negotiations with the Taliban are the only way to end the war, which the Obama administration is committed to winding down. Afghan leaders are openly calling on Islamabad to bring Afghan insurgents under its influence to the negotiating table, so Pakistan is seen as unlikely to open another front against militants it has so far avoided antagonizing.

The Haqqanis have not carried out attacks against the Pakistani state, unlike other militant factions behind a bloody wave of domestic terror that the Pakistani army is struggling to counter.

Critics believe the Pakistani army wants to use the Haqqanis to help to secure its interests in Afghanistan when the U.S. leaves. The military has a long history of using Islamic militants as proxies in Afghanistan and India. The Haqqanis are perhaps the most powerful faction in North Waziristan, which is also home to Pakistani militants and Al Qaeda operatives from around the globe.

Pakistan has no interest in seeing Afghanistan implode in violence. But it also believes it has the right to demand that any post-U.S. regime in Kabul accommodate its interests -- and its overriding desire is to see an Afghan regime that is hostile to India, which the Pakistani military sees as the country's principal threat, not Islamist militancy.

Pakistan's army, which controls Afghan policy, appears to believe Washington has limited options in forcing it to act.

U.S. officials have made veiled hints that they could launch unilateral operations against the Haqqanis inside North Waziristan. However, even limited operations by U.S. troops would be a red line for Pakistan, risking domestic turmoil in a country still vital in the anti-terror fight despite the current rancor.

Washington could also cut civilian and military aid to Pakistan or make handouts strictly contingent on Pakistani progress against the Haqqanis. But isolating nuclear-armed Pakistan, which is buckling under militant violence and an anemic economy, would be also be risky.

Pakistan also believes it has leverage over the United States: Islamabad currently allows the U.S. to truck much of its Afghan war supplies over its soil into the landlocked country. While that will ebb as America cuts troops, the route is still very valuable to Washington.

The fresh round of U.S. pressure on Pakistan began hours after the killing of the final member of the militant team that staged a 22-hour assault on the U.S. Embassy in Kabul. American officials in the region and in Washington accused the Haqqani network of carrying out the attack, and in the same breath said the group was being helped by Islamabad.

Previously, they had mostly said only that "elements" of the government had "links" to the group.

U.S. officials have not publicly linked the Haqqanis or Pakistan's spy agency to Monday's assassination in Kabul of Burhanuddin Rabbani, a former Afghan president and the head of an American-backed effort to seek peace with the Taliban. But Afghan officials have done so.

In his final congressional testimony before retiring next week, Mullen said Pakistan's government has chosen to "use violent extremism as an instrument of policy."

"By exporting violence, they have eroded their internal security and their position in the region. They have undermined their international credibility and threatened their economic well-being," he said.

Mullen said there was a "proxy connection" between Pakistani intelligence services and the Haqqanis.

American officials have not publicly offered evidence or details to back up the allegations of the Haqqani network's involvement in Afghanistan or Pakistan's links to it. Keeping things vague allows Washington to apply public pressure on Pakistan while allowing it leeway later on if Islamabad changes course.

The Americans also often admit that a bad relationship with Islamabad is better than none.

The Obama administration has tried to secure Islamabad's cooperation in the war by stressing its long-term commitment to Pakistan. The U.S. has promised the government $7.5 billion in aid for civilian projects over five years, and focused on the common interests it has with Islamabad in fighting militants that have killed many thousands of Pakistani soldiers and civilians in recent years.

American officials lined up to praise the army in 2009 and 2010 for launching offensives against militants in border areas. Many publicly backed the army's line that it lacked the troops to move into North Waziristan while tied up elsewhere in the tribal region.

But few are now prepared to speak up for the Pakistani military, especially after Usama bin Laden turned out to have been living in an army town close to Islamabad rather than some cave along the lawless border. That greatly added to suspicion in the U.S. that the Pakistanis were not to be trusted in the fight against extremism.

Critics of the army point out that the country is paying a price for not moving in force in North Waziristan.

Pakistani Taliban factions that are also based there have carried out hundreds of attacks against the Pakistani army and civilian targets over the last four years.

Some analysts say the army is making a slow strategic shift and does realize the need to sever links with jihadist groups. They say tactical reasons are behind the reluctance to move into North Waziristan -- that the army lacks the resources to hold areas already cleared of militants.

Moreover, they say, a half-strength operation risks cementing links between the Afghan Taliban and Pakistani militants, creating a force that could overwhelm security forces around the country.