"Today at 1:30 am (2230 GMT) in the Zaporizhzhia area... there was an attempt to break through our defences," Defence Minister Sergei Shoigu said.

"The enemy was stopped and retreated after heavy losses," Shoigu said, adding that the Ukrainian forces had used 150 armoured vehicles in their offensive.

Already this week, Russia has said it has repelled several assaults by Ukrainian forces.

Ukrainian officials have said their forces were ready for a long-expected counteroffensive but that there would be no formal announcement when it begins.

"Plans like silence," said Deputy Minister of Ukraine Ganna Malyar said. "The offensive will not begin on social media."