Kyiv has for months said it is preparing a major counteroffensive, hoping to reclaim territory lost since Russia launched its military operation in February 2022.

Donetsk is one of four Ukrainian territories that Russia annexed in September, along with Lugansk, Zaporizhzhia and Kherson.

Russia's defence ministry reported that on Sunday, in the south of the region, "the enemy launched a large-scale offensive in five sectors of the front".

"A total of six mechanised and two tank battalions of the enemy were involved," it said in a Telegram post, adding that Ukrainian troops had hit "the most vulnerable, in their opinion, sector of the front".

"The enemy did not achieve their tasks, they had no success."

On Monday, a Moscow-appointed official, Vladimir Rogov, said Kyiv's forces had launched a large scale attack on the southern Zaporizhzhia region.

"This morning, the Ukrainian forces launched an attack on a bigger scale than yesterday... the situation is alarming," Rogov said, according to the official TASS news agency.

Russia's defence ministry posted what it said was a video of the battle in Donetsk, showing Ukrainian armoured vehicles coming under heavy fire.

The conflict, which has dragged on for more than 15 months, escalated this week with increased attacks on both sides of the border.

In a video published Sunday, the Ukrainian army appeared to call on soldiers to stay silent and said there would be no announcement on the start of the long-awaited offensive.

The same day, the governor of Russia's Belgorod region on the border with Ukraine said fighting was ongoing on his side of the frontier and acknowledged that pro-Ukrainian forces had taken Russians prisoner during cross-border clashes.