Russia claimed victory in a months-long battle for Mariupol's Avostal steel plant, taking it a step nearer to its goal of controlling Ukraine's Donbas region, and halted gas exports to Finland in an escalation of an energy payment dispute with Western nations.

Russia also launched what appeared to be a major assault to seize the last remaining Ukrainian-held territory in Luhansk, one of two provinces that make up the southeastern Donbas region and where Russian-backed separatists already controlled swathes of territory before the Feb. 24 invasion.

The last Ukrainian forces holed up in Mariupol's smashed Azovstal steelworks surrendered on Friday, Russia's defense ministry said, ending the bloodiest siege of the war.

"The territory of the Azovstal metallurgical plant ... has been completely liberated," the ministry said in a statement, adding that 2,439 defenders had surrendered in the past few days, including 531 in the final group.

Hours earlier, Ukrainian President Volodymyr Zelenskyy said Ukraine's military had told the last defenders at the steelworks they could get out and save their lives. The Ukrainians did not immediately confirm the figures on Azovstal.

Ukraine's General Staff of Armed Forces did not comment on Russia's claim in its morning update on Saturday.

The end of fighting in Mariupol, the biggest city Russia has captured so far and the main port for the Donbas, gives Russian President Vladimir Putin a rare victory in the invasion after a series of setbacks in nearly three months of fighting.

Putin says Russian troops are engaged in a "special military operation" to demilitarise Ukraine and rid it of radical anti-Russian nationalists. Western countries call it an unprovoked war of aggression.

Victory in Mariupol gives Russia complete control of the Sea of Azov and an unbroken stretch of territory in eastern and southern Ukraine.

The Red Cross said it had registered hundreds of Ukrainians who surrendered at the Mariupol steel plant as prisoners of war and Kyiv says it wants a prisoner swap. Moscow says the prisoners will be treated humanely, but Russian politicians have been quoted as saying some must be tried or even executed.

Thousands of people in Ukraine have been killed and urban areas have been shattered in the war. Almost a third of Ukraine's people have fled their homes, including more than 6 million who have left the country.

Gas dispute

Meanwhile, Russia raised the stakes in an energy dispute with Western countries.

Russia's Gazprom halted gas exports to neighbouring Finland on Saturday after it refused to agree to Russian demands to pay for Russian gas supplies in roubles because of Western sanctions imposed over the invasion of Ukraine.

The move comes days after Finland and Sweden decided to apply to join the NATO military alliance, a decision spurred by the Ukraine war.

"Gas imports through Imatra entry point have been stopped," Finnish gas system operator Gasgrid Finland said in a statement on Saturday.

Finnish state-owned gas wholesaler Gasum and Gazprom also confirmed the flows had stopped.

Gasum, the Finnish government and individual gas consuming companies in Finland have said they were prepared for a shutdown of Russian flows and that the country will manage without.

Most European supply contracts are denominated in euros or dollars and Moscow cut off gas to Bulgaria and Poland last month after they refused to comply with the new payment terms.

Offensive in Luhansk

Russia launched what appeared to be a major assault to seize remaining Ukrainian-held territory in Luhansk.

Luhansk regional governor Serhiy Gaidai said in a social media post early on Saturday that Russia was trying to destroy the city of Sievierodonetsk, with fighting taking place on the outskirts of the city.

"Shelling continues from morning to the evening and also throughout the night," Gaidai said in a video post on the Telegram messaging app.

Despite losing ground elsewhere in recent weeks, Russian forces have advanced on the Luhansk front. Capturing Luhansk and Donetsk provinces would allow Moscow to claim a victory after announcing on March 25 that the Donbas region was now its focus.

The city of Sievierodonetsk and its twin Lysychansk across the Siverskiy Donets River form the eastern part of a Ukrainian-held pocket that Russia has been trying to overrun since mid-April after failing to capture Kyiv.

The Russian military also said on Saturday it had destroyed a major consignment of Western arms in Ukraine's Zhytomyr region, west of Kyiv, using sea-launched Kalibr cruise missiles.

Reuters could not independently verify the report, which also said Russian missiles had struck fuel storage facilities near Odesa on the Black Sea coast and shot down two Ukrainian Su-25 aircraft and 14 drones.