Mr. Bolton's latest testimony occurred amid ongoing congressional concern about the impact of Mr. Khan's activities.

Many lawmakers, in particular some Democrats on the House International Relations Committee, are upset with what they see as unanswered questions about Mr. Khan.

Others point to what they call Pakistani President Pervez Musharraf's lack of concern, and media reports of Pakistani government involvement in his network. Democratic Congressman William Delahunt said "I'm concerned that the United States, put aside any partisan differences here, is going to be embarrassed as being complicit with this charade, when it's time to get it out on the table."

Mr. Bolton says the administration has not sought access to Mr. Khan, something that angers some members of Congress who think U.S. anti-terror efforts demand that such contacts take place.

Mr. Bolton responded this way to suggestions that the United States, in the words of New York Democrat Gary Ackerman, is "turning a blind eye" possible involvement by Pakistani government officials. "The issue is the extent to which, if at all, the top levels of the government of Pakistan were involved in his activities, and as I say, we have no evidence to that effect," he said.

Undersecretary Bolton says the administration continues to try to unravel the various branches of the Khan network, adding he thinks there will be a "dramatic impact" on the international market in trafficking of weapons of mass destruction.

He says Libya's decision to give up its weapons of mass destruction programs is proof the administration's policies are working and a clear example for other countries such as North Korea and Iran.

On North Korea, Mr. Bolton defended the administration's approach, including the multi-party talks underway.

However, some lawmakers insist that the situation regarding Pyongyang's weapons program is worse now than it was before. California Congressman Adam Schiff said "I don't deny North Korea is a very tough problem. It is and I'm not sure there is any perfect policy. But I don't think there is any question but [that] we are at a worse stage now with North Korea than we were three years ago."

On Iran, Mr. Bolton faced questions about how aggressive the administration is in dealing with Tehran's nuclear program. California Democrat Brad Sherman repeated charges the administration is not doing enough to put pressure on Iran. "The administration believes in enforcing the law, but both you and your predecessors have never used ILSA [Iran-Libya Sanctions Act] to impose sanctions. Not on TOTAL, which is investing in Iran now, not on the Japanese where you wouldn't even have to impose them, you would just have to express disapproval and they would pull back. The attitude has been shovel the money to Iran, and bomb Iraq," he said.

Mr. Bolton disagreed, saying the administration made strong "high-level" representations, in private and public, trying to persuade Tokyo not to go ahead with a $2.8 billion loan for a key oil project in Iran.