Washington: Pakistan must take steps to prevent a safe haven for terrorist networks and cross-border attacks from its territory, said CIA chief Leon Panetta, who on Thursday expressed fears that there is a danger that Pakistani nuclear weapons could fall into "wrong hands".

"I strongly urge those in Pakistan to take steps to do whatever they can to prevent these kind of cross-border attacks and to prevent the safe havens that do exist on the Pakistani side of the border," said Panetta, who has been nominated as the next Defence Secretary by President Barack Obama.

Panetta told lawmakers at his Senate confirmation hearing that the bilateral ties with Pakistan is one of the most "complicated and frustrating relationship".

"It's critical because they are a nuclear power, and there's the danger that those nukes could wind up in the wrong hands," Panetta underlined.

He said the relationship was was "very complicated" because Islamabad maintains "relationships with certain terrorist groups, that they continue to not take aggressive action with regard to these safe havens, and that are concerned about the sovereignty results and criticisms of the US when in fact my view is that the terrorists in their country are probably the greatest threat to their sovereignty".

At the same time, he underlined the importance of maintaining the relationship. He said it was critical because vital supply lines to Afghanistan go through their country.

"We've got to do everything we can to try to strengthen that relationship so that both of us can work to defend both of our countries," Panetta said.

Panetta backs Afghan drawdown

Leon Panetta said that he backed President Barack Obama's call for withdrawing significant numbers of American troops from Afghanistan starting in July.

US Senate Armed Services Committee Chairman Carl Levin asked Panetta, who has served Obama as CIA chief, whether he agreed with the president that the pending withdrawal should be "significant."

"I agree with the president's statement," said Panetta, whose comment seemed to put him at odds with outgoing US Defence Secretary Robert Gates's call for a modest draw-down.

Quizzed later on the same issue by Senator John McCain, Panetta declined to state "specific numbers" but said that conditions on the ground would shape that decision.

"We have made progress with regards to security in that country, albeit fragile and reversible," said Panetta, who was virtually sure to win US Senate confirmation.