The proof of the pudding is in the eating, a US official was quoted as saying, indicating Washington's disapproval of the release of Khan who is accused of running a clandestine nuclear proliferation ring.
"Pakistan has given us some initial commitments but we're going to be following the situation very closely. The important thing is that they know we are still very serious about this individual," the official was quoted by The Washington Post which did not name him.
The US feels that A Q Khan remains a significant proliferation risk.
72-year-old Khan was pardoned in 2004 by President Pervez Musharraf who, however, put the scientist under house arrest.
Khan last year retracted his confession of having passed nuclear know-how to North Korea and Iran, saying it had been made under pressure. He was released by a Pakistani court on Friday.
