Ukrainian President Volodymyr Zelenskiy said Russia had begun the "Battle of Donbas" in the east and a "very large part of the entire Russian army is now focused on this offensive".

"No matter how many Russian troops they send there, we will fight. We will defend ourselves," he said in a video address on Monday.

Zelenskiy's chief of staff, Andriy Yermak, assured Ukrainians their forces could hold off the offensive in "the second phase of the war".

"Believe in our army, it is very strong," he said.

There was no immediate comment from Russia's defence ministry on the latest fighting. The governor of the Russian province of Belgorod said Ukrainian forces had struck a border village wounding one resident.

Ukrainian media reported a series of explosions, some powerful, along the front line in the Donetsk region, with shelling taking place in Marinka, Slavyansk and Kramatorsk.

Blasts were also heard in Kharkiv in the northeast, Mykolaiv in the south and Zaporizhzhia in the southeast while air raid sirens were also going off in main centres near the front line, officials and media said.

Reuters was not immediately able to verify the reports.

Ukraine's top security official, Oleksiy Danilov, said Russian forces attempted to break through Ukrainian defences "along almost the entire front line of Donetsk, Luhansk and Kharkiv regions".

Driven back by Ukrainian forces in the north, Russia has refocused its ground offensive in the two eastern provinces known as the Donbas, while launching long-distance strikes at other targets including the capital, Kyiv.

Donbas has been the focal point of Russia's campaign to destabilise Ukraine, starting in 2014 when the Kremlin used proxies to set up two separatist "people's republics" in the ex-Soviet state. It is also home to much of Ukraine's industrial wealth, including coal and steel.

Ukraine's general staff said Russian forces aimed to establish full control over the Donetsk, Luhansk and Kherson regions, while intensifying missile strikes in west Ukraine.