Spokesman Richard Boucher's comments appeared to contradict a claim by Pakistani President Pervez Musharraf that Washington had not provided specific evidence of Pakistani nuclear scientist Abdul Qadeer Khan's proliferation activities until October.

Musharraf, in an interview published in Tuesday's editions of The New York Times, said "if they knew it earlier, they should have told us.

"Maybe a lot of things would not have happened," the Pakistani leader said in a reference to the disgraced Pakistani nuclear hero's confession to providing nuclear secrets to US foes North Korea, Libya and Iran.

Boucher told reporters the United States has had "longstanding concerns about proliferation that could come from Pakistan.

"We've discussed non-proliferation issues with Pakistan repeatedly over a long period of time and it's been an issue of concern to us and to President Musharraf as well," he said.

"We have talked to them about the potential for onward proliferation from Pakistan," Boucher said. "We have talked to them at different moments about different issues that might have arisen that we might have learned about.

"So it's not a single moment of information. It's rather an ongoing dialogue that covered both the general concerns that we had about possibilities, and then from time to time, pieces of information that related to different aspects of things that we might have encountered or known were going on.

"Certainly, our non-proliferation dialogue with Pakistan goes back much farther than (October)," Boucher said.

"I'd say that Pakistan has taken this matter seriously over time and particularly with regard to the current investigation and what they're doing to make sure that Pakistan is not a source of proliferation," he said. "We welcome that progress and will continue to work with Pakistan as Pakistan investigates and also works itself with the international community."

In the interview with the Times, Musharraf said there had been suspicions since February 2000 that Khan was sharing information with other countries.

"We got some suspicious reports through the security agencies... But we didn't catch them red-handed," Musharraf said.

He also said he was concerned that investigating Khan, considered a national hero for developing Pakistan's nuclear bomb, could provoke a political backlash.

US Secretary of State Colin Powell said Monday that he has told Musharraf he must pull up the nuclear smuggling ring led by Khan "by its roots."

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