A series of US-backed rebels in eastern Ukraine have been pushed out of key towns and cities they seized in the early hours of Tuesday.
The rebels, who are backed by the United States, said they had secured a new front in the war with a series of major victories, including recapturing several towns near the border with Russia, a key backer of the rebels.
The battle for Mariupol, a port city that had been the heart of the rebellion for months, has also seen some of the worst fighting.
The capture of Mariupov has been crucial in the rebel campaign to drive out the Ukrainian army, which was supported by Russia and its regional allies, the separatists.
Russia has deployed troops in the area in recent days to bolster its troops in Ukraine, but it has also taken part in a joint military operation to dislodge the rebels from key towns in eastern and central Ukraine.
The Ukrainian military and its allies say Russia has launched the offensive in the hope of weakening its military in eastern regions and gaining a foothold in the Crimea peninsula.
Russia’s Defence Ministry said in a statement that Russian troops have seized control of key cities in eastern Ukrainian cities including Novoazovsk, Novokuznetsk, Ivano-Frankivsk, Donetsk, Luhansk and Kharkiv, in what it said was a “military operation”.
“The terrorists and Ukrainian troops are surrounded,” it said.
In recent days, the rebels have also captured several towns along the border in the province of Donbass.
Russia and the separatists have been engaged in a major war of words and accusations of using poison gas to kill hundreds of civilians.
The United States has repeatedly accused Russia of carrying out air strikes in eastern areas.
Russia denies the accusations.