Ukraine's army chief says Ukrainian forces have taken control of 1,000 square kilometers of the western Russian region of Kursk. Intense fighting has continued in the border region since Kyiv launched the incursion last week.

Commander-in-Chief Oleksandr Syrskyi made the claim on Monday. This is the largest incursion by Ukrainian forces since the Russian invasion began in February 2022.

Reuters news agency quoted the acting governor of Kursk, Alexei Smirnov, as saying the incursion was about 12 kilometers deep and 40 kilometers wide -- less than half of the area Ukraine claims to have captured.

Russia's defense ministry announced on Tuesday that it destroyed 12 of Ukraine's drones over the Kursk region.

US think tank the Institute for the Study of War said Russian President Vladimir Putin and the military command likely incorrectly assessed that Ukraine lacked the capability to take the initiative in the fighting and allowed Kyiv to succeed in making the surprise attack.