Pakistan has told the United States it will put some curbs around freed scientist Abdul Qadeer Khan to prevent him from becoming a renewed nuclear proliferation threat, a US official said Monday.

However, the official said the United States wanted more as well as "solid" assurances from Pakistan that he will not be such a threat after a court released Khan, dubbed the father of Pakistan's atomic bomb, from house arrest on Friday.

"Whatever it is that they decide in terms of putting additional restrictions on his (Khan's) movements, we will have to see," the State Department official told reporters on the condition of anonymity.

"I understand that he has to notify (his) government 48 hours in advance if he wants to travel outside of Islambad. That's one of the things they've communicated to us," according to the official.

"I'm sure there's more that the Pakistanis can do, and we expect and hope that they will do more to make sure that he is no longer a proliferation risk."

The official said Anne Patterson, the US ambassador, received assurances from Pakistani government officials during a meeting in Islamabad at the weekend.

The official said Washington was still skeptical.

"We want to make sure these assurances are solid and that they can explain to us as to how they plan to do so," the official said.

Khan, 72, was freed by a Pakistani court on Friday.

He had been under a virtual house arrest in Islamabad since February 2004, when he publicly confessed to sending nuclear secrets to Iran, Libya and North Korea, although he later retracted his remarks.

The State Department's acting spokesman Robert Wood said Richard Holbrooke would also discuss the Khan case in Islamabad after he arrived there Monday on his first assignment as the new US special envoy to Pakistan and Afghanistan.

"I would suspect the issue would come up in his conversations with the Pakistanis," Wood told the daily press briefing.

As part of his mission to bring peace and stability to Afghanistan, Holbrooke will meet government and military officials, leaders of civil society, and the business community, Wood said.

He will travel next to Afghanistan before ending his trip in India, he added.

A US official, speaking on the condition of anonymity, said Holbrooke will also ask about Pakistan's investigation into India's claims that a Pakistani militant group was behind the deadly siege in Mumbai in late November.

The Mumbai bombings are not part of Holbrooke's brief, but he needs to follow up on an attack in which six US nationals died, the official said.