A defiant North Korea, facing pressure to scrap a nuclear weapons program, warned the United States today that it would take unspecified "tougher counteraction" if Washington did not accept talks on the issue.

Breaking its silence about the U.S. disclosure last week that the communist state had acknowledged it was secretly pursuing a uranium-reprocessing program, North Korea said Washington must "opt for reconciliation and peace."

"If the U.S. persists in its moves to pressurize and stifle [North Korea] by force, the latter will have no option but to take a tougher counteraction," the official party daily Rodong Sinmun said in a statement carried by the North's official Korean Central News Agency.

In Moscow, U.S. Undersecretary of State John R. Bolton sought to step up the diplomatic pressure on North Korea, saying its uranium-enrichment program was "real and dangerous."

Bolton was in Russia as part of a tour of Asian and European capitals to enlist support to halt North Korea's arms program by diplomatic means. He was going on to London and Paris.

He told reporters that he had passed on to Russian officials confidential information about the North Korean program and that he expected the issue to be discussed when President Bush and Russian leader Vladimir Putin meet Saturday in Mexico.

"I should tell you that our very careful, very deliberate, very prudent assessment of the information we have is enough to convince us that this program is real and dangerous, no matter what the North Koreans say," he said.

"What we've said is that they are seeking production-scope capability to produce weapons-grade uranium and that that effort is a violation of the nonproliferation treaty and a grave cause of concern to us, to the states in the region and to the world as a whole," Bolton said.

On Monday, North Korea's number two leader, Kim Yong Nam, told South Korea's visiting unification minister that the North was ready for dialogue.

The U.S. ambassador in Seoul, Thomas C. Hubbard, said today that Washington sought to preempt a crisis through diplomacy, but that North Korea had exhausted its credibility with the secret nuclear program, which violated a previous negotiated settlement.

"We have very little basis for trust in North Korea, very little basis for confidence that further dialogue will lead to a solution," he said.