TOKYO, Japan — Japan said Friday it was following with "serious concern" indications that North Korean troops are in Russia ahead of their possible deployment in Ukraine.

"The Japanese government is... monitoring the deployment of soldiers by North Korea to Russia with serious concern, including the possibility that they may join in Russia's aggression against Ukraine," Chief Cabinet Secretary Yoshimasa Hayashi said.

"The recent development of military cooperation between Russia and the DPRK (North Korea), including with this development, not only invites the further deterioration of the situation in Ukraine, but it is seriously worrisome also from the perspective of its impact on the security of the region surrounding Japan," Hayashi told reporters.

"Japan will continue to collect and analyze relevant information and continue to work with the international community for the full implementation of relevant Security Council resolutions and the realization of a just and lasting peace in Ukraine as soon as possible," he said.

He added: "Russia's invasion of Ukraine is an outrageous act that shakes the very foundations of the international order."

Seoul's spy agency has said that around 3,000 North Korean soldiers are currently in Russia training, likely to deploy to the frontlines against Ukraine soon, with thousands more troops to be sent by December.

NATO and Washington have confirmed that Pyongyang's soldiers are now training in Russia, warning that if they were to join the fight against Kyiv, it could mark a dangerous escalation of the grinding war

Ukraine said Thursday that North Korean troops have arrived in the "combat zone" in Russia's Kursk border region, where Moscow has struggled to push back Ukrainian forces for months.

North Korea — with whom Russia signed a mutual defense pact — is already widely believed to be arming Moscow for its invasion but troops on the ground would mark a new escalation in the conflict.