Washington ? North Korea’s cryptic comment that it has “more powerful” weapons highlights U.S. worries that Pyongyang may be hiding potent biological weapons like smallpox or advanced nuclear weapons.

While U.S. officials say they believe North Korea probably doesn’t have thermonuclear weapons, finding out what’s in the reclusive dictatorship’s arsenal will require inspections.

“North Korea is a closed system,” Defense Secretary Donald Rumsfeld said this week. “Our intelligence on it is imperfect.”

Inspecting and disarming North Korea will be one topic President Bush takes up today and Saturday when he meets with the leaders of China, Japan and South Korea. Pyongyang has thwarted or rejected past weapons inspection efforts by the International Atomic Energy Agency and other groups.

The United States broke off talks with North Korea earlier this month after Pyongyang admitted having a clandestine program to enrich uranium for nuclear weapons. That program violates a 1994 pact in which North Korea agreed to halt its nuclear weapons development in exchange for two civilian nuclear reactors and other aid.

The U.S. is demanding that North Korea eliminate its nuclear weapons program before any talks resume, while North Korea has said America must drop its “hostile policy.”

“The North Koreans have admitted cheating,” said Gary Samore, a former State Department and National Security Council official involved in negotiating the 1994 pact. “It’s hard to see on what basis one could proceed to negotiate another agreement with them.”

In an announcement today, North Korea said it wanted a “nonaggression treaty” with the United States to resolve security concerns on the Korean peninsula. A State Department official said the United States would comment later on the proposal.

It is unclear what the North Koreans meant by the “more powerful” remark, said U.S. defense and intelligence officials, who said there’s no public evidence North Korea has anything stronger than one or two relatively crude nuclear bombs.