Ukraine's President Volodymyr Zelenskyy has offered encouragement to Ukrainian soldiers deployed in an incursion into Russian territory.

Zelenskyy visited the northeastern Ukrainian region of Sumy bordering Russia's Kursk region on Friday.

He told the troops that the Kursk operation has greatly helped Ukraine in motivating those who supply the country with weapons and support packages from abroad.

Almost two months into the major incursion into Russia's Kursk region, Ukraine still controls parts of Kursk despite counterattacks by Russian forces.

Meanwhile, media outlets in Ukraine quoted military sources on Friday as saying six North Korean officers were among 20 soldiers killed in a Ukrainian missile attack on Russian-controlled areas near Donetsk in the country's east on Thursday.

The reports also quoted Russian social media posts as saying the North Koreans were undergoing training by Russia when the missile struck on Thursday.

Russia and North Korea have been strengthening military cooperation as Pyongyang provides Moscow with munitions.