By Guardian - Dan Sabbagh in KyivSat 10 Aug 2024 15.44 BST

Russia’s defence ministry claimed it prevented Ukraine from advancing further on the fifth day of the unprecedented attack into the province of Kursk, though there were reports of regional power outages after an electricity substation was hit. Fighting said to be continuing, with reports of power outages near nuclear power station, despite Moscow’s claim

Fighting was said to be taking place in three villages between seven and 11 miles from the international border – Ivashkovskoye, Malaya Loknya and Olgovka – similar locations to where Ukraine is estimated to have advanced previously.

In a morning statement, the defence ministry said it had “thwarted the attempts of the enemy’s mobile groups to get to the depth of the Russian territory” and there were no other significant reports to the contrary.

Russia’s FSB domestic security agency also imposed a “counter-terrorism” regime on Kursk and two neighbouring oblasts, Bryansk and Belgorod, giving the authorities sweeping powers to lock down an area and impose controls on communications.

Ukrainian leaders and its military have refrained from commenting on its attempt to take the war directly on to Russian soil, though on Friday afternoon Ukraine’s president, Volodymyr Zelenskiy, implied the operation had been discussed at a meeting of the defence staff.

A report from Gen Oleksandr Syrskyi had, Zelenskiy said, “also covered our defensive actions in the directions from which Russia launched attacks on Ukrainian territory” – an apparent reference to the unprecedented attack launched this week.

Ukrainian regular forces burst over the border north-east of Sumy on Tuesday in a surprise attack on a lightly defended sector of the frontline and advanced about 13 miles, capturing towns and villages and destroying a Russian convoy 25 miles from the border, causing dozens of casualties.

It was the first time that Ukraine had attacked with regular forces inside Russia’s territory – a tactic that at one point might have provoked considerable anxiety among Kyiv’s western backers for fear of escalation from the Kremlin.

Overnight, however, Ukraine received a vote of confidence from the US, which announced a further $125m (£98m) package of military aid, including artillery shells, rockets and anti-aircraft Stinger missiles.

Ukraine appeared to have inflicted another surprise on the Russians when a short video emerged on Saturday of five soldiers bearing a Ukrainian and Georgian flag outside a club building in Poroz, two miles inside the border in Belgorod province – and about 45 miles south of this week’s incursion.

Though there was no immediate sign that this was any more than a stunt, it served to demonstrate the vulnerability of Russia’s border away from the combat zones inside Ukraine, where the war has been raging since Moscow’s full-scale invasion began more than two years ago.

Alexey Smirnov, the acting governor of the Kursk region, said on Friday that a fire had broken out in a transformer substation after debris from a Ukrainian drone had crashed into the facility. Power was out in some frontline areas, he added, including Kurchatov, where the Kursk nuclear power station is based.

Russian officials said there was no impact on the nuclear plant on Saturday, which was said to be operating normally, though there were reports it was being reinforced. Few believe Ukraine’s military has the capacity to get close to the nuclear plant, which is roughly 30 miles from the nearest fighting as the crow flies.

Nevertheless, Rafael Grossi, the head of the International Atomic Energy Agency, said on Friday he had been monitoring the battlefield situation. “At this juncture, I would like to appeal to all sides to exercise maximum restraint in order to avoid a nuclear accident,” Grossi said.