More than 10,000 North Korean troops have arrived in Russia to support its war against Ukraine, with a "significant number" in the frontline areas including Kursk, South Korea's defence ministry said on Tuesday.

The remarks came hours after the Pentagon said that there were at least 10,000 North Korean soldiers in Kursk, near the Ukraine border, but could not corroborate reports that they were engaged in combat.

Ukraine's intelligence agency has also said that about 12,000 North Korean troops, including 500 officers and three generals, were already in Russia, with training taking place at five military bases.

"We understand that more than 10,000 North Korean troops are currently in Russia, and a significant number of them have moved to the frontline areas including Kursk," Jeon Ha-kyou, a spokesperson for Seoul's defence ministry, told a briefing, citing intelligence authorities.

But Jeon said he had no information when asked whether the North Koreans were engaged in combat, and about a South Korean media report citing an unnamed government official that as many as 40 North Korean troops had been killed on the battlefield.