Cohen

WASHINGTON (CNN) -- U.S. Secretary of Defense William Cohen says American officials are "concerned" about reports that traces of plutonium have been detected at suspected nuclear weapons production facilities in North Korea.

Under a 1994 agreement, North Korea agreed to freeze its nuclear weapons program in exchange for nuclear power plants and other economic help from the United States and other countries. But American officials say North Korea is balking on inspections designed to verify compliance with the accord.

"This is not something that can go on indefinitely. This is a subject matter which needs to be addressed," Cohen said. "We are going to need inspections of the site or sites that might be involved. Absent ... agreement to do that, it will call very much into question the agreed (nuclear) framework itself."

On Wednesday, high-level delegations from the United States and North Korea held 12 hours of talks in Pyongyang. The State Department said afterward that it was not satisfied with the responses it has received from the Koreans.

N. Korea asks for compensation

During those talks, the North Koreans reportedly asked the United States to pay $300 million for the right to inspect an underground site that is a suspected nuclear weapons facility. State Department spokesman James Rubin confirmed that the North Koreans had raised the compensation issue.

"We dismissed those ridiculous numbers out of hand," Rubin said. "We are not going to pay compensation to confirm that they are living up to their obligations under this important nuclear agreement."

Both Rubin and Cohen were asked about published reports that soil and water samples taken from a site in North Korea indicated the present of plutonium. Cohen declined to confirm the reports, saying the information was classified, though he did say the United States was "concerned" about those reports.

Rubin would only say that "we have credible evidence, or we wouldn't be in this situation of demanding this kind of direct discussion on this subject."

Reuters contributed to this report.