KABUL: Secretary of Defense Leon Panetta said Thursday that the United States was running out of patience with Pakistan over safe havens of insurgents who attack US troops across the border in Afghanistan.

Panetta arrived in Afghanistan on Thursday to take stock of progress in the war and discuss plans for the troop draw down, even as violence spiked in the south.

Panetta spoke after talks with Afghan Defence Minister Abdul Rahim Wardak on the latest leg of an Asian tour that has taken him to Pakistan's arch-rival India, but not Islamabad in a sign of how dire US-Pakistan relations are.

He singled out the Haqqani network, a Taliban and al Qaeda-linked faction believed to be based in Pakistan's lawless tribal district of North Waziristan.

“It's an increasing concern that Haqqani safe havens still exist on the other side of the border. Pakistan has to take action from allowing terrorists in their country to attack our forces on the other side of the border,” he said.

“We are reaching the limits of our patience here,” he added.

Afghan and US officials have blamed the Haqqani network for some of the deadliest attacks of the 10-year war, including a brazen 18-hour assault on Kabul in April -- the biggest to hit the Afghan capital in a decade.

Panetta said that in talks with Pakistan, the United States had made “very clear, time and time again,” the need to crack down on Haqqani militants.

Pakistan has resisted US pressure to launch a major offensive against the network in North Waziristan, arguing that it is too overstretched in the fight against local Taliban to take on an enemy that poses no threat to Pakistan.

Independent analysts have suggested that Pakistan is not capable of defeating the Haqqanis, a well organised and disciplined force that can command thousands of fighters.

Speaking to troops gathered at the airport in Kabul, Panetta kept up the drumbeat on Pakistan that began two days earlier during his visit to India. Panetta told the troops that ''we have every responsibility to defend ourselves and we are going to make very clear that we are prepared to take them on and we've got to put pressure on Pakistan to take them on as well.''

A senior US official acknowledged Thursday that the recent increase in drone strikes on insurgents in Pakistan is due in part to frustration with Islamabad.

Despite pressure from US commanders, Pakistan remains reluctant to go after insurgents, particularly the Haqqani network, which was likely responsible for recent deadly attacks on US forces.

Panetta's explicit description of frustration, while visiting neighboring India, appeared to signal a somewhat tougher stance and a suggestion that the US is becoming even more willing and quick to strike terrorist targets inside Pakistan.

The defense secretary also joked with troops at the Kabul airport about the US strike that killed an al-Qaeda leader Monday, saying, ''the worst job you can get these days is to be a deputy leader in al-Qaeda, or for that matter a leader.''

Insurgents in Afghanistan more organised

Making his fourth trip to the war zone, Panetta acknowledged the increase in attacks and that the insurgents appear to be much more organized.

But he insisted that the overall level of violence was down, and that commanders had expected the uptick.

Panetta said he wants to get an assessment of the situation from the top US commander, Marine Gen. John Allen, and see how confident he is about Nato's ability to confront the threats both from the Taliban and the Haqqani network, which often operates from safe havens in Pakistan.

''I think it's important to make sure we are aware of the kind of attacks they're going to engage in ... as we go through the rest of the summer,'' Panetta told reporters traveling with him during a stop in New Delhi, India, on Wednesday.