The US military's top officer bluntly accused Pakistan on Thursday of ‘exporting’ violent extremism to Afghanistan through proxies and warned of possible US action to protect American troops.

In a scathing and unprecedented US condemnation of Pakistan, Admiral Mike Mullen said the country's main intelligence agency ISI was actively supporting Haqqani network militants blamed for an assault on the US embassy in Kabul last week.

"The Haqqani Network, for one, acts as a veritable arm of Pakistan’s Inter-Services Intelligence agency," Mullen told the US Senate Armed Services Committee.

The Haqqani network is probably the most dangerous faction in the Afghan Taliban. A CIA asset turned Al-Qaeda ally, the United States in the 1980s funneled arms and cash to the Haqqani faction to counter Soviet forces.

ISI backs Haqqani militants: Mullen

Mullen said Haqqani militants – with ISI backing – this month carried out a truck bombing on a NATO base in Afghanistan that wounded 77 Americans; assaulted the US embassy and NATO headquarters in the Afghan capital; and in June staged an attack on the InterContinental hotel in Kabul.

The admiral's tough language follows a series of stern warnings from top US officials on Pakistan's failure to crack down on the Haqqani network, raising the possibility of unilateral US action.

"If they keep killing our troops that would not be something we would just sit idly by and watch," Mullen said of the Haqqani insurgents.

US warning comes strong

The Central Intelligence Agency already carries out drone bombing raids on Al-Qaeda and other militants in Pakistan's northwest tribal areas, strikes which US officials do not explicitly acknowledge.

The US warnings carry particular weight in the aftermath of the American raid that killed Al-Qaeda chief Osama bin Laden on May 2 in Abbottabad, an operation that angered and embarrassed Pakistani leaders.

US officials did not alert Islamabad in advance of the nighttime operation by Navy SEAL commandos, fearing that officials might tip off bin Laden's circle.

Another Osama raid on cards?

Defense Secretary Leon Panetta, appearing at the same Senate hearing, expressed frustration over Haqqani sanctuaries in Pakistan and renewed a vow that the United States would safeguard its troops.

Panetta declined to say what steps the government might take – amid speculation the United States might expand drone strikes to a wider area or even stage an operation similar to the bin Laden raid.

Panetta said Pakistan needed to take action not only on the Haqqani network but also to cooperate on tracking down senior extremists identified by Washington and to bolster a military campaign against militants inside its borders.

Pakistan this week promised action against the Haqqani network if the United States provided sufficient intelligence, but denied the Al-Qaeda-linked Taliban faction operated on Pakistani soil.

‘Violent extremism Pak policy’

Mullen told senators that Pakistan was jeopardizing its partnership with Washington as well as its regional influence by ‘choosing to use violent extremism as an instrument of policy’. He added: "By exporting violence, they have eroded their internal security and their position in the region."

While Pakistan has maintained ties to some militants as a hedge to counter its arch-foe India, the gamble has proved a failure, he said.

‘Bring peace in Kashmir’

To defuse underlying tensions in the region, Panetta said both India and Pakistan needed to reach a peace settlement over the disputed region of Kashmir.

"It's tough politically in both areas, but in the end, we are never going to achieve stability in that region until the issues between Pakistan and India are resolved," he said.

Mullen to step down

In his last apperance before the Senate committee as he prepares to step down this month as chairman of the US Joint Chiefs of Staff, Mullen defended his efforts to build a dialogue with Pakistan's military despite mixed results.

"Some may argue I have wasted my time, that Pakistan is no closer to us than before – and may now have drifted even further away. I disagree," he said.