Asian and European nations urged North Korea (search) on Saturday to return immediately to nuclear disarmament talks as concerns grew that the communist state was preparing to test an atomic bomb.

Pyongyang (search) could expect the security assurance and financial assistance it seeks in return for a decision to abandon nuclear weapons development and return to six-nation negotiations, South Korean Foreign Minister Ban Ki-moon (search) said at a news conference.

The ministers, wrapping up a two-day Asia-Europe Meeting in Japan, called on North Korea "to return to the negotiating table of the six-party talks without any further delay ... to achieve the denuclearization of the peninsula in a peaceful manner through dialogue."

Those talks include the two Koreas, the United States, China, Japan and Russia.

A U.S. defense official, who spoke only on condition of anonymity, said Friday that photos by U.S. spy satellites show North Korea making moves that could be construed as preparations for an underground nuclear test.

Japan's Defense Agency also said it had information that the communist nation might be preparing for a nuclear test.

Japanese Foreign Minister Nobutaka Machimura (search) stressed that Tokyo had not confirmed whether the preparations were real but acknowledged that the communist country posed an increasing threat.

"It is very difficult to ascertain any of this," he said.

"In the meantime, nuclear and missile development has likely been proceeding steadily," since North Korea broke off the talks last June, Machimura said.

Although North Korea has claimed it has nuclear weapons, an actual test would be a first. U.S. intelligence and other estimates put the number of their weapons between one and six.

Machimura told Ban on Friday that Japan might bring North Korea's nuclear ambitions before the U.N. Security Council if the North continued to boycott talks. That would be a first step toward possible sanctions against the reclusive communist regime.

North Korea's leaders have said they would consider sanctions a "declaration of war."

Jitters over North Korea's nuclear activities, as well as the test-firing earlier this week of a short-range missile toward Japan, set off a flurry of meetings at the Japan conference.

Japanese, South Korean and Chinese officials met later Saturday and agreed to strengthen their efforts to bring North Korea back to the six-party negotiations.

The ASEM forum includes the 25 EU states, the European Commission, the 10-country Association of Southeast Asian Nations, Japan, China and South Korea.