(Adds Polish president in Ukraine, to address parliament)
* Ukraine rules out ceasefire, concessions
* Russia launches assault in Luhansk
* Russia stops Finland gas flows over payment dispute
* Polish president in Ukraine to address parliament Sunday
By Tom Balmforth, Pavel Polityuk and Terje Solsvik
KYIV/OSLO, May 21 (Reuters) - Ukraine ruled out a ceasefire or concessions to Moscow on Saturday as Russia intensified an offensive in the eastern Donbas region and stopped providing gas to Finland.
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 Feb. 24 invasion, but Moscow wants to seize the last remaining Ukrainian-held territory in Donbas.
"The situation in Donbas is extremely difficult," Ukrainian President Volodymyr Zelenskiy said in his nightly address. The Russian army was trying to attack the cities of Sloviansk and Sievierodonetsk, but Ukrainian forces were holding off their advance, he said.
Earlier, Zelenskiy told local television that although the fighting would be bloody, the end would come only through diplomacy and that the Russian occupation of Ukrainian territory would be temporary.
Zelenskiy adviser Mykhailo Podolyak ruled out agreeing to a ceasefire and said Kyiv would not accept any deal with Moscow that involved ceding territory. He said making concessions would backfire on Ukraine because Russia would hit back harder after any break in fighting.
"The war will not stop (after concessions). It will just be put on pause for some time," Podolyak, Ukraine's lead negotiator, told Reuters in an interview in the heavily guarded presidential office. "They'll start a new offensive, even more bloody and large-scale."
Recent calls for an immediate ceasefire have come from U.S. Defense Secretary Lloyd Austin and Italian Prime Minister Mario Draghi.
The end of fighting in Mariupol, the biggest city Russia has captured, 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.
The last Ukrainian forces holed up Mariupol's vast Azovstal steelworks surrendered on Friday, Russia said.
Full control of Mariupol gives Russia command of a land route linking the Crimean Peninsula, which Moscow seized in 2014, with mainland Russia and areas of eastern Ukraine held by pro-Russia separatists.
Ukrainian forces in the separatist-controlled regions of Luhansk and Donetsk said on Saturday they had repelled nine attacks and destroyed five tanks and 10 other armored vehicles in the previous 24 hours.
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, they said.
Russian troops destroyed a bridge on the Siverskiy Donets River between Sievierodonetsk and Lysychansk, Luhansk regional governor Serhiy Gaidai said. There was fighting on the outskirts of Sievierodonetsk from morning through the night, he said on the Telegram messaging app.
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.
Russia's state gas company, Gazprom, said it halted gas exports to Finland, which refused Moscow's demands to pay in roubles for Russian gas after Western countries imposed sanctions over the invasion.
Finland and Sweden applied this week to join the NATO military alliance.
Finnish state-owned gas wholesaler Gasum, the Finnish government and individual gas consuming companies in Finland have said they were prepared for a shutdown of Russian flows.
Most European supply contracts are denominated in euros or dollars. Last month, Moscow cut off gas to Bulgaria and Poland after they refused to comply with the new terms.
Western nations also have stepped up weapons supplies to Ukraine. On Saturday, Kyiv got another huge boost when U.S. President Joe Biden signed a bill to provide nearly $40 billion in military, economic and humanitarian aid.
Moscow says Western sanctions, along with arms deliveries for Kyiv, amount to a "proxy war" by the United States and its allies.
The Russian military said 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.
Thousands of people in Ukraine have been killed in the war that has displaced millions and shattered cities.
Zelenskiy said he stressed the importance of more sanctions on Russia and unblocking Ukrainian ports in a call with Italian Prime Minister Mario Draghi on Saturday.
Polish President Andrzej Duda was in Ukraine preparing to address the Ukrainian parliament on Sunday. Duda, who also met with Zelenskiy in Kyiv last month, is the first head of state to address parliament in person since the invasion, his office said.
British Prime Minister Boris Johnson delivered a May 3 address to lawmakers via videolink.
(Reporting by Natalia Zinets, Max Hunder, Tom Balmforth in Kyiv, David Ljunggren in Ottawa, Lidia Kelly in Melbourne and Reuters bureaux, Writing by Madeline Chambers, Richard Pullin and Doina Chiacu; Editing by Frances Kerry, Timothy Heritage, David Gregorio and Nick Zieminski)