Japan's top government spokesperson has expressed deep concerns about reports that North Korean troops are in Russia.

Chief Cabinet Secretary Hayashi Yoshimasa's remarks to reporters on Friday came after a senior US official said North Korean soldiers are being trained in eastern Russia, possibly to fight against Ukraine.

Ukraine's defense ministry said the first batch of North Korean troops had arrived in combat zones.

Hayashi said the Japanese government is closely monitoring the situation, including the possibility that North Korea might join Russia's invasion of Ukraine.

He also said that the recent development in the deepening military cooperation between Moscow and Pyongyang not only deteriorates the situation in Ukraine, but also raises serious concerns for regional security around Japan.

Hayashi said the government is collecting and analyzing information. He added that Japan will continue to work closely with the international community to fully implement related United Nations Security Council resolutions, and achieve fair and long-lasting peace in Ukraine as soon as possible.