US Assistant Secretary of State Christopher Hill is surrounded by reporters upon arrival at Narita international airport, near Tokyo, Monday, September 4, 2006

Envoy Christopher Hill is on a mission to confer with Japanese and Chinese officials as concerns are rising in Tokyo that North Korea will carry out an explosive nuclear test.

America's top regional envoy says the United States and Japan will take "concrete action" to implement a U.N. resolution meant to stop North Korea from conducting further missile tests. The resolution requires U.N. member states to prevent Pyongyang from obtaining technology, components or money to make missiles or weapons of mass destruction.

After meetings with Japanese Foreign Ministry officials Tuesday, Assistant Secretary of State Christopher Hill said the United States is open to talking with North Korea about its nuclear weapons program. But he said other countries must attend such discussions.

"We can look at other such meetings. We think the time for organized multilateral diplomacy is now, especially in Northeast Asia," he said.

North Korea has refused for the past 10 months to return to six-nation talks about ending its nuclear programs.

North Korea claims to have nuclear weapons and fears are growing that the communist state could be preparing to test a nuclear explosive device for the first time.

Foreign Ministry spokesman Tomohiko Taniguchi says if Pyongyang conducts a test Japan would consider it a significant crisis beyond that seen when North Korea test-fired missiles in July.

"It is going to be posing even more serious danger to the international community than the multiple launches of the missiles," he said.  "And so we have to start up a close coordination between the Cabinet Office and the Ministry of Foreign Affairs and between Washington, D.C. and Tokyo yet again."

Japan's Defense Agency chief has said that Washington and Tokyo are cooperating to gather intelligence about a possible nuclear test. He says that there is no sign that test is imminent.