The government is stepping up efforts with the United States and South Korea to urge countries that accept workers from North Korea to stop in order to cut off financing to Pyongyang's nuclear and missile development programs, sources close to the matter said.

Defying U.N. Security Council resolutions and warnings from the international community, North Korea has conducted four nuclear tests, most recently in January, as well as rocket launches using ballistic missile technology.

At a time when Japan, along with the United States and South Korea, remain on alert for a possible fifth nuclear test by the North, the sources said on Saturday that the three allies have been trying since spring to root out one of the sources of North Korea's finances: foreign currency earned by North Korean workers abroad.