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.
He singled out the Haqqani network, a Taliban and Al-Qaeda-linked faction believed to be based in 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.