The U.S. says there will be a "firm response" as Ukrainian forces holding ground in Russia's Kursk region now face North Korean troops "engaging in combat."

North Korean soldiers have engaged in combat operations against Ukraine alongside their Russian allies, South Korea's spy agency said on Wednesday. Seoul's confirmation comes amid growing global criticism as the two countries strengthen military ties,

North Korean troops have begun engaging in combat operations alongside Russian forces, the U.S. State Department said on Tuesday, expressing concern that Russia had turned to North Korea for soldiers to fight in its war with Ukraine.

The attack ended a two-month pause in missile strikes on Kyiv, which had only been hit by Russian drones during that period.