Seoul "will not stand by but respond firmly in collaboration with the international community," South Korea said.

Secretary of Defense Lloyd J. Austin III confirmed on Wednesday that North Korea had sent troops to Russia to join the fight against Ukraine, a major shift in Moscow’s effort to win the war. Mr. Austin called the North’s presence a “very serious” escalation that would have ramifications in both Europe and Asia.

Kim Jong-un’s soldiers are expected to arrive in Russia’s frontline region of Kursk today, in a worrying sign of the military cooperation between the two nations. As many as 12,000 troops deployed from North Korea to fight for Vladimir Putin are set to pour into the southwestern border region,