Defense Secretary Leon Panetta said on Thursday the United States was reaching the limits of its patience with Pakistan because of the safe havens the country offered to insurgents in neighboring Afghanistan.

It was some of the strongest language used by a U.S. official to describe the strained ties between Washington and Islamabad.

"It is difficult to achieve peace in Afghanistan as long as there is safe haven for terrorists in Pakistan," said Panetta, speaking in the Afghan capital Kabul where he arrived for talks with military leaders amid rising violence in the war against the Taliban and a spate of deadly attacks, including a NATO air strike said to have killed 18 villagers.

"It is very important for Pakistan to take steps. It is an increasing concern, the issue of safe haven, and we are reaching the limits of our patience," he told reporters.

The United States has long pushed Pakistan to do more to help in the war against militancy, but the relationship has received a series of blows, not least by a unilateral U.S. raid into Pakistan to killOsama bin Laden last year that humiliated Islamabad.

Panetta also urged Pakistan to go after the Haqqani militant network, one of the United States' most feared enemies in Afghanistan, and said Washington would exert diplomatic pressure and take any other steps needed to protect its forces.

"It is an increasing concern that safe havens exist and those like the Haqqanis make use of that to attack our forces," he said.

"We are reaching the limits of our patience for that reason. It is extremely important for Pakistan to take action to prevent (giving) the Haqqanis safe havens, and for terrorists to use their country as a safety net to conduct attacks on our forces."

The comments came as Washington appears to be looking to other allies in the region for help in the face of Pakistan's foot-dragging. Panetta arrived in Kabul after a visit to India, Pakistan's old enemy, where he urged New Delhi to take a more active role in Afghanistan.

NATO has signed an agreement with three countries to the north of Afghanistan for land routes as the U.S.-led alliance begins a withdrawal of its forces from the country next year.

NATO chief Anders Fogh Rasmussen said earlier this week the "reverse transit" deal was signed with Uzbekistan, Kazakhstan and Kyrgyzstan.

Pakistan closed the shorter and cheaper routes through its territory last year to protest a cross-border NATO air attack that killed 24 Pakistani soldiers. Discussions to reopen the Pakistan routes have stalled.

Other irritants in U.S.-Pakistan ties include drone attacks in the lawless areas of Pakistan near the Afghan border where several militant groups operate but seen by many Pakistanis as a violation of sovereignty.

Washington says the attacks are crucial to attack militants and four days ago, a U.S. drone strike in northwest Pakistan killed al Qaeda's second-ranking leader, Abu Yahya al-Libi. It was the biggest blow to the militant group since the killing of bin Laden.

Tensions between Washington and Islamabad have also flared because a Pakistani doctor accused of helping the CIA find bin Laden was jailed for 33 years for treason last month. REUTERS