The lightning incursion into Russia unfurled on Aug. 6 when thousands of Ukrainian troops crossed Russia's western border in a major embarrassment for Putin's military.

The United States and Western powers, eager to avoid direct confrontation with Russia, said Ukraine had not given advance notice and that Washington was not involved, though weaponry provided by Britain and the U.S. was reported to have been used on Russian soil.

Hawkish Kremlin aide Nikolai Patrushev dismissed the Western assertions in an interview with the Izvestia newspaper.

"The operation in the Kursk region was also planned with the participation of NATO and Western special services," he was quoted as saying, without offering evidence.

"Without their participation and direct support, Kyiv would not have ventured into Russian territory."

The remarks by one of the Kremlin's influential "Cold War warriors" implied that Ukraine's first acknowledged incursion into sovereign Russian territory since Moscow sent its forces into Ukraine in 2022 carried a high risk of escalation.

"Washington's efforts have created all the prerequisites for Ukraine to lose its sovereignty and lose part of its territories," Patrushev said.

Ukraine's top commander said on Thursday that Kyiv had set up a military commandant's office in the part of Russia's Kursk region where he said his forces were still advancing, even as Moscow's troops stepped up its offensives in Ukraine's east.

TRYING TO AVOID NATO-RUSSIA CONFLICT

While the Ukrainian attack has revealed weaknesses in Russia's defences and changed the public narrative of the conflict, Russian officials said what they cast as a Ukrainian "terrorist invasion" would not change the course of the war.

Russia has been advancing for most of the year in the key eastern sector of the 1,000-km (620-mile) front line and has vast numerical superiority. It controls 18% of Ukraine.

The U.S. so far deems the surprise incursion a protective move in which it is appropriate for Kyiv to use U.S. equipment, officials in Washington said.

But they also expressed worries about complications as Ukrainian troops push further into enemy territory.

One U.S. official, speaking on condition of anonymity, said that if Ukraine started taking Russian villages and other non-military targets using U.S. weapons and vehicles, it could be seen as stretching the limits Washington has imposed, precisely to avoid any perception of a direct NATO-Russia conflict.

Britain said on Thursday that weaponry it had given to Ukraine could be used inside Russia to help Kyiv defend itself, and a British source said British Challenger 2 tanks were thought to have been used on Russian territory.

Russia's defence ministry has published footage that it said showed a Russian drone destroying a U.S.-made Stryker armoured combat vehicle in the Kursk region.

In Moscow, one lawmaker said the Ukrainian incursion and the presence of Western military equipment on Russian soil had brought World War Three a step closer.