We turn now to events in Russia, and Moscow is introducing strict new security measures in three regions, border Ukraine in response to this week's surprise incursion by Ukrainian forces into the Kursk region. There are also concerns from the UN nuclear agency, which has urge both sides to exercise maximum restraint as fighting moves closer to a nuclear power plant. Fighting has been raging since Ukraine's army launches surprise incursion this week, with reports suggesting that they could be operating up to 30 kilometers inside Russia with intense fighting close to the town of Suja. Frances Reid reports.

Filmed from a dashboard, then uploaded onto social media. Abandoned and burned Russian troop carriers in the Kursk region. The person who posted these pictures has since been arrested, but the BBC has verified the footage. Ukrainian assaults on Russia started on Tuesday and seem to have come as a surprise. Several villages have been seized. This footage from the Ukrainian military is purported to show Russian soldiers surrendering. Reports suggest the focus is happening just north of Ukraine, with fighting thought to be concentrated near the town of Suja. Moscow says up to a thousand Ukrainian troops are involved in a region which contains a nuclear plant. The UN nuclear agency has urgent restraint, but the astonishing images of Russian jets bombing Russian territory seem to run counter to the Kremlin's official narrative that the war in Ukraine has been going to plan. President Putin holding a meeting with his Security Council. It's calling the Ukrainian incursion, Terrorism. And Russia is now bringing in reinforcements. But no act goes unpunished. A Russian missile attack attack on a supermarket in the Eastern Donetsk region has proved fatal, although it's not clear whether it was a direct response to Ukraine's cross-border assault.

Anja was there. We started climbing out of the window, she says. We were barely outside when everything started burning. I started running one way, my colleague ran the other way. Ukrainian emergency services have been searching through the rubble for signs of life. No respite for those living this reality. Frances Reid, BBC News.

We can speak now to our Kyiv correspondent, James Waterhouse. James, good to have you with us. Are we any clearer as to understanding what Ukraine's aim here is in this surprise incursion?

I think if you look at the pictures you've just seen, I think that reflects what Ukraine was after in the short term, at least. Russia is putting more and more resources into trying to contain this offensive. We've seen a lot of men and machinery moved in the direction of Kursk and neighboring regions along the border as Ukrainian forces seemingly take more and more territory. They've even been frustrated in that endeavor with one convoy certainly being destroyed. Now we have what's being described as a counterterror operation, a counterterrorism operation by the Kremlin Hamlin, which gives powers for cars to be searched, for phone tapping, for the movement of vehicles and people to be extremely limited. This is something Russia is having to take seriously. I think for Ukraine, the hope is that it will relieve pressure on the front lines on its own territory, where it is Russia continuing to advance. It's important not to understate with how significant this week has been, because this has been a war, certainly for Ukraine, of grueling familiarity. Here we've had this strange inversion, where you have the sight of Russian civilians reacting with fear and anger in some cases, and and leaving their homes.

This is something that we see across Ukraine continually, and it really has shaken things up in that respect. Ukraine has increased the stakes. And in this war of territory, where Russia often talks about wanting Ukraine to concede land in exchange for peace. Well, now Ukraine controls a chunk of Russian territory. It really is a high stakes move, and it could... We are still talking about Ukrainian forces that are stretch. So it's been an extraordinary few days.

It has indeed. Briefly, if you don't mind, you were just mentioning there how high stakes all of this is. Do we have any intelligence as to understanding whether Ukraine can continue this incursion in Russia whilst also trying to hold the front lines in its own country?

We don't. At the moment, colonels and generals and senior officers are being incredibly tight-lipped about this operation. But what we understand, what we can glean from the front lines is that the issue of military aid after political disagreements among the West, that's partially been resolved. Equipment is arriving on the front line, but Ukraine is grappling with its own domestic challenge of mobilizing enough men. So it was seen as counterintuitive at best to send clearly elite fighters into Russia in this way. But if it forces some concession politically or in a military sense, by Moscow, then it will be seen as worth it.

Our Ukraine correspondent there, James Waterhouse, speaking to us from Kyiv. As ever, great to have you reporting. Thank you, James.