"We have not received any information on North Korea to put us on high alert," said Chief Cabinet Secretary Yasuhisa Shiozaki, the government spokesman.

Takemasa Moriya, the number two at Japan's Defense Agency, indicated the current situation was not an emergency.

"We are strengthening our efforts in gathering information," Moriya told reporters.

Japan was conducting monitoring operations "through appropriate use of ships and airplanes," he said.

In Washington, a senior intelligence official said Wednesday that the United States had detected movement in North Korea at one of several suspected sites, which could indicate a bomb test.

South Korean newspaper Dong-a Ilbo, which gave no source, speculated that the North might stage its test early next week or early next month.

North Korea said Tuesday it will test its first atomic bomb at an unspecified time in the future, raising the stakes in a standoff with the United States over its nuclear program.

Japan is particularly sensitive to developments in North Korea as Pyongyang fired a missile over Japan's main island in 1998.