Russia said on Wednesday that it was fighting intense battles against Ukrainian forces which had penetrated Russia's Kursk region in one of the largest incursions into Russia since the Ukraine war began in February 2022.

Russia has advanced this year after the failure of Ukraine's 2023 counteroffensive to achieve any major gains, and has taken 420 square kilometres (162 square miles) of territory from Ukrainian forces since June 14, Sergei Shoigu, head of Russia's security council, has said.

Ukraine struck back on Tuesday - and the battles continued through the night into Wednesday as Ukrainian forces pushed to the north west of the border town of Sudzha, 530 km (330 miles) southwest of Moscow, Russia's defence ministry said.

The defence ministry said that it was continuing to fight Ukrainian units "in the areas of the Kursk region directly adjacent to the Russian-Ukrainian border."

"Air strikes, missile forces, artillery fire and active actions of units covering the State border of the grouping of troops in the Kursk direction prevented the enemy from advancing deep into the territory of the Russian Federation."

It said the fighting was continuing, adding that it had already destroyed 50 armoured vehicles, including seven tanks, eight armoured personnel carriers, three infantry fighting vehicles and 31 armoured combat vehicles in the area.

Sudzha is the last operational trans-shipping point for Russian gas exports to Europe via Ukraine. Just 60 km away to the northeast is Russia's Kursk nuclear power station.

Ukraine has not commented on the events. Russia sent reserves to help shore up Russian defences.

The battles around Sudzha come at a crucial juncture in the war: Ukraine is losing territory and Kyiv is deeply concerned that U.S. support could drop off if Donald Trump wins the November election in the United States.

Trump has said he would end the war, so both Russia and Ukraine are keen to gain the strongest possible bargaining position on the battlefield while pinning down Russian forces and showing the West that it can still mount major battles.

Shoigu said on Tuesday that the window for peace was narrowing and that the longer it took for Kyiv to begin to talk about terms, the costlier the peace would be for the Ukrainian people.