Expressing concern over the release of A Q Khan from house arrest, the United States told Pakistan that the disgraced nuclear scientist continues to be a "serious threat" to proliferation of nuclear weapons technology.

US Special Representative for Pakistan and Afghanistan Richard Holbrooke, who is currently on a visit to Islamabad [Images], certainly raised the issue pertaining to A Q Khan, State Department spokesperson Robert Wood said on Wednesday.

"We'll have to see how things go. We certainly don't want to see that nuclear black market up and operating. We'll continue to follow the issue very closely," he said.

Last month, the US had slapped sanctions against Khan and his network for their alleged involvement in the proliferation of nuclear technologies to countries like North Korea, Iran and Libya.

Wood said Pakistan has given the assurance that it would ensure that Khan does not involve in such activities after his release.

Meanwhile, Wood said Holbrooke would continue with the remaining part of his trip to the region, despite the latest suicide bombing attack in Kabul by the Taliban [Images].