The outgoing U.S. spy chief has made a surprise stop in Pakistan, part of an apparent effort to mend ties with Islamabad.

Senior U.S. and Pakistani officials say CIA Director Leon Panetta arrived in Pakistan Friday for talks with the Pakistani Army Chief General Ashfaq Parvez Kayani. He also met with Pakistani spy chief Lieutenant General Ahmed Shuja Pasha.

It is Panetta's first trip to Pakistani since the May 2nd raid by U.S. commandos that killed terror mastermind Osama bin Laden. The operation angered Paksitani officials, who were not told about the mission until it was over.

Panetta has been nominated to replace current U.S. Defense Secretary Robert Gates. Gates is retiring at the end of the month.

On Thursday, Panetta told lawmakers in Washington that the U.S. must do everything it can to strengthen the “complicated” relationship with Pakistan. He said it is clear that the U.S. cannot succeed in Afghanistan if it is not succeeding in neighboring Pakistan, controlling terrorist safe havens and cross-border attacks.

Meanwhile, most of the U.S. experts sent to help train the Pakistan's military in counterinsurgency have left the country at Islamabad's request.

U.S. Vice Admiral Michael LeFever said Friday the United States has almost completed the pullout, though he did not give specific numbers. He said in a statement from the U.S. Embassy in Islamabad that the U.S. remained ready to help Pakistan if asked.

Pakistani security officials, who refused to be named, said about 90 of the approximately 130 U.S. trainers had been sent home.

Pakistan asked for the drawdown last month after U.S. commandos infiltrated Pakistan and killed al-Qaida leader Osama bin Laden.

Relations between the two countries have been strained since the May 2 raid.

Pakistani leaders criticized the action, saying it was a violation of Pakistan's sovereignty. They also warned against future unilateral actions.

Pakistan received $2.7 billion in security-related assistance from the United States in the fiscal year that ended last October. It is the third-largest recipient of U.S. security aid and reimbursements, after Afghanistan and Israel.