Russia intensified an offensive in the Donbas region of eastern Ukraine on Saturday and stopped providing gas to Finland, escalating Moscow's dispute with the West over energy payments.

After ending weeks of resistance by the last Ukrainian fighters in the strategic southeastern city of Mariupol, Russia is waging what appears to be a major offensive in Luhansk, one of two provinces in Donbas.

Russian-backed separatists already controlled swathes of territory in Luhansk and the neighbouring Donetsk province before the February 24 attack, but Moscow wants to seize the last remaining Ukrainian-held territory in Donbas.

Ukrainian forces in those eastern separatist-controlled regions said on Saturday they had repelled nine attacks and destroyed five tanks and 10 other armoured vehicles in the previous 24 hours. They said that as of 9pm local time, there was still fighting in four unspecified locations.

Russian forces were using aircraft, artillery, tanks, rockets, mortars and missiles along the entire front line to attack civilian structures and residential areas, the Ukrainians said in a Facebook post. At least seven people were killed in the Donetsk region, the forces said.

Ukrainian President Volodymyr Zelenskiy told local television that while the fighting would be bloody, and victory difficult, the end would come only through diplomacy.

"For them, all these victories – the occupation of Crimea or Donbas – is very temporary. And all this will return – since this is our territory," he said on Saturday.

The end of fighting in Mariupol, the biggest city Russia has captured so far, could be crucial to its ambitions in Donbas. It gives Russian President Vladimir Putin a rare victory after a series of setbacks in nearly three months of combat.

Russia's state gas company, Gazprom, said it halted gas exports to Finland after it refused to agree to Russian demands that it pay for Russian gas in roubles because of Western sanctions imposed over the invasion of Ukraine.

Finland and Sweden applied this week to join the Nato military alliance, a decision spurred by the Ukraine war. (Reuters)