KYIV (Kyodo) -- About 50,000 Russian and North Korean troops deployed in Russia's Kursk region are fighting against Ukrainian forces, a senior government official told Kyodo News on Tuesday.

The Ukrainian official said fierce fighting has broken out and North Korean soldiers have been killed in some areas, as the United States officially acknowledged for the first time on Tuesday that Pyongyang had sent troops to support Russia's war.

As Ukrainian forces have seized control of part of the Kursk region after a cross-border attack, fears are growing that Russia could launch a large-scale offensive to retake it as soon as possible. North Korean soldiers are believed to have received training in Russia.

North Korea said Tuesday it had ratified a defense cooperation treaty with Russia, including a provision for mutual assistance if either country comes under attack.

The Comprehensive Strategic Partnership treaty was signed in June by North Korean leader Kim Jong Un and Russian President Vladimir Putin. Russia, which launched a full-scale invasion of Ukraine in February 2022, has already ratified the pact.