[Source: Reuters]

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.

The incursion has come as a shock to Russia, nearly 2 1/2 years after President Vladimir Putin sent his army into Ukraine.

Kursk’s regional acting governor, Alexei Smirnov, said thousands of residents had been evacuated.

The White House said the United States – Ukraine’s biggest backer – had no prior knowledge of the attack.

Pentagon spokesperson Sabrina Singh said Ukraine’s move on the Kursk region was consistent with U.S. policy.

She said Ukraine’s move was not escalatory because “Ukraine is doing what it needs to do to be successful on the battlefield.”

Russia’s defence ministry said on Thursday that the army and the Federal Security Service (FSB) had halted Ukraine’s advance and were battling Kyiv’s units in the Kursk region.

The Ukrainian military has remained silent on the Kursk offensive, though President Volodymyr Zelenskiy praised the Ukrainian army on Thursday for its ability “to surprise” and achieve results. He did not explicitly reference Kursk.

Some Russian bloggers said Ukraine’s forces were pushing towards the Kursk nuclear power station, which lies about 60 km (37 miles) northeast of Sudzha.

Yuri Podolyaka, a popular Ukrainian-born, pro-Russian military blogger, said that there were intense battles about 30 km from the Soviet-era nuclear plant, which supplies a large swathe of southern Russia with power.