Washington, Sep. 2 : Expressing concerns over the Lahore High Court's decision to end all security restrictions on disgraced Pakistani scientist Dr. A. Q. Khan the United States has warned that Dr. Khan still remains a `serious proliferation risk'.

Interacting with media persons here, State Department spokesperson Ian Kelly said the Obama Administration is keeping a close watch on developments regarding Dr. Khan, and it has already expressed its concerns to Islamabad.

`Our concerns over the potential for proliferation activities by Mr. Khan are well known to the Pakistani government. We believe that he remains a proliferation risk," Kelly said.

"We're following this closely," he added.

A former UN weapons inspector and nuclear specialist has also raised questions over ending all security restrictions on the Pakistani scientist, who had confessed in 2004 that he had `illegally' provided nuclear know-how's to countries such as Libya, Iran and North Korea.

The President of the Institute for Science and International Security (ISIS), David Albright, said it was mistake to `free' Dr. Khan.

"It is a mistake to remove restrictions on a man who cannot be `trusted. There is a risk that Khan, who retracted his 2004 confession, will now be able to offer nuclear know-how to scientists from Iran, North Korea and any other rogue countries feared to be pursuing a nuclear weapons program," The Dawn quoted Albright, as saying.

In January 2004, Khan had confessed to having been involved in a secret international network of nuclear weapons technology proliferation from Pakistan to Libya, Iran and North Korea.

It is believed that Khan and his network were one of the worst proliferators of nuclear technology that could be used to develop nuclear weapons.

US Secretary of State Hillary Clinton had also expressed concerns when Islamabad High Court eased restrictions on Khan in February this year. (ANI)