Russian forces were battling Ukrainian troops for a third day on Thursday after they smashed through the Russian border in the Kursk region, an audacious attack on the world's biggest nuclear power that has forced Moscow to call in reserves.

In one of the biggest Ukrainian attacks on Russia since the war began in February 2022, around 1,000 Ukrainian troops rammed through the Russian border in the early hours of Aug. 6 with tanks and armored vehicles, covered in the air by swarms of drones and pounding artillery, according to Russian officials.

Heavy fighting was reported near the town of Sudzha, where Russian natural gas flows into Ukraine, raising concerns about a possible sudden stop to transit flows to Europe.