North Korean troops based in Russia are operating drones across the border into Ukraine on reconnaissance missions, the Ukrainian military said on Thursday, the first time Kyiv has reported a battlefield role for North Koreans in months.

Thousands of North Korean troops fought last year alongside Russian forces in Russia’s Kursk region against Ukrainian troops who mounted their biggest incursion into Russia of the war.

Ukraine later said the North Koreans were withdrawn after suffering heavy casualties. Ukrainian troops were pushed out of Kursk in March this year and have not reported engaging North Koreans on the battlefield since.

In a statement on Facebook, the Ukrainian General Staff said North Korean units were now operating drones from the Kursk region to find Ukrainian military positions and help target rocket strikes on Ukraine’s adjacent Sumy region.

“The Defence Forces of Ukraine have intercepted communications between North Korean drone operators and personnel of the Russian army,” it said.

North Korea and Russia have signed a mutual defence pact and upgraded military cooperation in the past two years. Ukraine and South Korea estimate Pyongyang deployed more than 10,000 troops to fight in the war against Ukraine in return for economic and military technology assistance.

South Korea’s defence minister said on Monday North Korea had probably received technical help from Russia for its submarine development.