WASHINGTON - The United States Tuesday reacted immediately after the lifting of restrictions on Pakistan's nuclear scientist Abdul Qadeer Khan, calling him a "serious proliferation risk." "We believe he remains a serious proliferation risk," State Department Spokesman Ian Kelly said in response to a question at the regular news briefing. He also indicated that the United States follows the scientists' activities "closely." "We have made clear to the Pakistani government our concerns about A.Q. Khan," the spokeman said. "We have frequent contacts with Pakistani officials." In answer to another question, the spokesman said he has not seen reports about an increase in Pakistani nuclear weapons. US-MADE MISSILES On reports about the modification of American-made decades-old missiles, the spokesman said he has seen the New York Times report. "We take the reports very seriously, we are engaged with the Pakistan government at highest level." He said the two countries have recently negotiated an agreement, in principle, about the inspection of US-supplied equipment. The administration has notified the Congress about the issue of possible modifications of military equipment provided to Pakistan. However, he said he is talking in general terms and not specifically about the missile issue concerning Pakistan. "I'm talking in general terms -- i am not aware of any representation (Richard) Holbrooke (the US envoy to Pakistan and Afghanistan) made to Pakistani officials" in recent visit on missile subject, he added.