By Deborah Haynes, security and defence editor

Details about the number of Ukrainian soldiers inside Russia remain unclear as commanders have deliberately stayed silent about a mission that was planned in secret.

But it is likely to be in the thousands, with elements from at least three well-equipped brigades on the ground, deploying tanks, armoured vehicles, artillery guns and drones.

Also hard to measure is how far the Ukrainian attackers have pushed, with Russian military bloggers saying they have penetrated up to around 12 miles from the Ukrainian border.

Videos, widely circulated on social media, purport to show Ukrainian soldiers raising the yellow and blue flag of Ukraine over Russian territory, including in the town of Sudzha and a settlement close to the Ukrainian border in the next-door region of Belgorod.

Under pressure, Russia has rushed in reinforcements and released footage of its military fighting back, but this is the sixth day of the Ukrainian offensive and battles are still raging.

Commenting on events, analysts have noted that it is the first time Russia has been invaded since Adolf Hitler in 1941.

But Ukraine’s attack is not the act of an aggressive power making a land grab.

Instead it is the counterintuitive action of a nation that was invaded by Vladimir Putin’s Russia a decade ago – with the capture of Crimea and parts of eastern Ukraine – and subsequently further devastated by Moscow’s full-scale invasion in February 2022.

It makes Kyiv’s counter invasion into Kursk just the latest – though arguably the most audacious – effort by Ukraine to repel Russian forces from inside its own sovereign territory.

Countering Russia’s much larger military means Ukrainian commanders have had to be innovative from the very start and be willing to take huge risks.

Two and a half years ago, outgunned and outnumbered Ukrainian troops – backed by Western weapons – defied the odds to prevent Russia from seizing Kyiv.

They then forced Russian invaders to withdraw from the whole of the north of Ukraine.

Months later, in September 2022, the Ukrainian side innovated once more.

It launched a surprise counteroffensive against Russian troops occupying the northeast of their country – just as Russia was encountering a simultaneous but much more widely anticipated counterattack in the south.

Ukraine recaptured swathes of territory in the Kharkiv region in that push. Its forces also continued the counterattack in the south, recapturing the southern city of Kherson.

However, Russian positions in other parts of southern Ukraine and across the Donbas in the east are far more entrenched and harder to defeat.

With Ukraine’s Western allies handing over increasingly lethal weapons such as tanks and long range missiles, anticipation did grow last year about a second Ukrainian counteroffensive.

But that effort faltered as combat units failed to penetrate heavily fortified Russian lines.

This time around, under the leadership of Colonel General Oleksandr Syrskyi, commander in chief of the Ukrainian armed forces, Kyiv has taken a different gamble.

Rather than prepare its troops to attempt once more to storm Russian positions inside Ukraine, leaders have chosen to send their soldiers into Russia, where the border is – perhaps surprisingly – far less well-defended.

It is very unlikely that Ukraine has the capacity or the desire to seize and hold much – if any – territory, but it has dealt a humiliating blow to Mr Putin and brought Russia’s bloody war in Ukraine far closer to home for the Russian people and their president.