In a rare large-scale and surprise attack on Russian territory, about 1,000 Ukrainian troops crossed the Russian border early on August 6 in tanks and armored vehicles, protected by swarms of drones and artillery in the air, according to Russian officials.

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

President Vladimir Putin called the Ukrainian attack “a major provocation”. The acting governor of the Kursk region, Alexei Smirnov, said thousands of residents had been evacuated.

The White House said the United States, Ukraine’s biggest backer, had no advance knowledge of the attack. The United States has asked Ukraine for details of its military targets, a U.S. official said on Thursday.

“They are operating in the area north of the border, where we have told them they can only use US-supplied weapons to defend themselves against Russian attacks,” the official noted.

The Russian Defense Ministry said on Thursday that the military and the Federal Security Service (FSB) had stopped the advance of the Ukrainian army and were fighting Ukrainian units in the Kursk region.

"Units of the Northern Group of Forces, together with the Russian FSB, continue to destroy armed formations of the Ukrainian army in the Sudzhensky and Korenevsky districts of the Kursk region, directly adjacent to the Russian-Ukrainian border," the ministry said.

President Volodymyr Zelenskyy praised the Ukrainian military on Thursday for its ability to “surge surprise” and achieve results. He did not mention Kursk specifically. Several Russian bloggers said Ukrainian forces were advancing toward the Kursk nuclear power plant, located about 60 kilometers northeast of Sudzha.

Yuri Podolyaka, a prominent pro-Russian military blogger of Ukrainian origin, said fierce fighting had broken out about 30 kilometers (19 miles) from the Soviet-era nuclear plant that supplies electricity to a large swath of southern Russia.

Ukraine's energy minister says gas transit through Sudzha is still going on, despite reports of fighting there. Most European Union countries have reduced their dependence on Russian gas, but Austria is one country that still gets the bulk of its gas via Ukraine.

Ukraine wants to limit advances by Russia, which now controls 18% of its territory, although the strategic significance of the border offensive remains unclear.

Russia's Security Council deputy chairman, Dmitry Medvedev, said the Ukrainian offensive was an attempt to force Russia to divert its strength away from the frontline and to demonstrate to the West that Ukraine could still fight.

Huy Hoang (according to TASS, Reuters, CNN)