The lightning incursion, the biggest into Russia by a foreign power since World War II, unfurled on August 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 military confrontation with Russia, said Ukraine had not given advance notice and Washington was not involved, though weaponry provided by Britain and the US is reported to have been used on Russian soil.
Influential veteran Kremlin hawk 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.
Ukraine's 10-day push has been the biggest invasion of Russia since World War II. (AP PHOTO)
"Without their participation and direct support, Kyiv would not have ventured into Russian territory."
The remarks implied that Ukraine's first acknowledged foray 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 said on Thursday it had installed a military commandant in the area it controlled, even as Russia intensified its offensives in Ukraine's east.
While the Ukrainian attack has revealed weaknesses in Russian defences and changed the public narrative of the conflict, Russian officials said Ukraine's "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 1000km front and has vast numerical superiority.
It controls 18 per cent of Ukraine.
After more than 10 days of fighting, Ukraine holds at least 450 square kilometres of territory, or less than 0.003 per cent of Russia.
But for Putin, the incursion crosses another red line.
Thousands of people have fled their homes in the Kursk region since Ukraine began its incursion. (AP PHOTO)
He said on Monday that Russia would deliver a "worthy response" beyond ejecting Ukraine's forces.
One Russian source told Reuters the incursion could embolden hardliners in Moscow who advocate a bigger war, but Putin's choice might not be easy.
He has sought to portray Europe's biggest war in seven decades both as a limited "special military operation" that need not upset daily Russian life and as a historic fight with a West that scorns Moscow's interests and seeks to dismember Russia.
The US, which rejects such allegations but says it cannot allow Russia to seize part of a sovereign neighbour, so far deems the surprise incursion a protective move that justifies the use of US weaponry, officials in Washington said.
But they also expressed worries about complications as Ukrainian troops push further into enemy territory.
One US official, speaking on condition of anonymity, said if Ukraine started taking Russian villages and other non-military targets using US 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 it said showed a Russian drone destroying a US-made Stryker armoured combat vehicle in the Kursk region.
Separately, the defence minister of Belarus, which Russia used as one launch point for the conflict in 2022, said there was a high probability of an armed provocation from Ukraine and that the situation at their common frontier was "tense".
