The State Department says the U.S. special representative for Pakistan and Afghanistan will speak with senior Pakistani authorities this week about the release of a Pakistani scientist who leaked nuclear secrets.

U.S. Ambassador Richard Holbrooke arrived in Islamabad, Pakistan, Monday for scheduled meetings with senior Pakistani government officials and representatives from the private sector and civil society.

The State Department said that Holbrooke spoke with foreign ministry officials Sunday and was assured that Abdul Qadeer Khan will not be a future proliferation risk.

Spokesman Robert Wood told reporters Monday that the U.S. "remains very disappointed" by Khan's release.

Wood says the United States will closely follow Khan's case. But he says Secretary of State Hillary Rodham Clinton has not raised the matter with her Pakistani counterpart.

Pakistan says Khan's international atomic smuggling network has been dismantled and cannot rise again.

The Associated Press contributed to this report.