The United States has concerns over what Russia might be doing to strengthen North Korean forces, including their nuclear capacity, Secretary of State Antony Blinken told reporters on Wednesday during his visit to Brussels.

Washington has said North Korean troops have been engaging in combat operations alongside Russian forces in its war against Ukraine.

Seoul's confirmation comes amid growing global criticism as the two countries strengthen military ties, after Pyongyang sent thousands of troops to Russia's Kursk region to aid its war with Ukraine.

"The National Intelligence Service estimates that North Korean troops dispatched to Russia have moved to the Kursk region over the past two weeks," the agency said in a statement.

They had completed deployment to the battlefield, and were already participating in combat operations, it added.

North Korean state media Tuesday said Pyongyang had ratified a mutual defense treaty with Russia signed in June by the two countries' leaders, providing for each to come to the other's aid in case of an armed attack.