As Russia wages offensive in Donbas, Ukraine rules out ceasefire or land concessions
Polish president offers Warsaw's backing in address to Ukraine's parliament
Updates from Day 88 of the invasion
Heaviest fighting focused around eastern cities of Sievierodonetsk, Lysychansk.
Polish president addresses Ukraine's parliament in Kyiv, reaffirms support.
Russia claims airstrikes in eastern Donbas region, southern Mykolaiv region.
Zelensky says 50 to 100 Ukrainians dying every day on war's eastern front.
Ukraine ruled out a ceasefire or any territorial concessions to Moscow as Russia stepped up its attack in the country's east and south on Sunday, pounding the Donbas and Mykolaiv regions with airstrikes and artillery fire.
Kyiv's stance has become increasingly uncompromising in recent weeks as Russia experienced military setbacks, while Ukrainian officials grew worried they might be pressured to sacrifice land for a peace deal.
"The war must end with the complete restoration of Ukraine's territorial integrity and sovereignty," Ukraine's presidential chief of staff, Andriy Yermak, said in a Twitter post on Sunday.
Ukraine's lead negotiator, Mykhailo Podolyak, ruled out any territorial concessions and rejected calls for an immediate ceasefire, saying it meant Russian troops would stay in occupied territories — which Kyiv could not accept.
"The [Russian] forces must leave the country and after that the resumption of the peace process will be possible," Podolyak said in an interview with Reuters on Saturday, referring to calls for an immediate ceasefire as "very strange."
Concessions would backfire because Russia would use the break in fighting to come back stronger, he said.
Recent calls for an immediate ceasefire have come from U.S. Defence Secretary Lloyd Austin and Italian Prime Minister Mario Draghi.
Polish president in Kyiv
Polish President Andrzej Duda offered Warsaw's backing, telling lawmakers in Kyiv on Sunday that the international community had to demand Russia's complete withdrawal and that sacrificing any of it would be a "huge blow" to the entire West.
"Worrying voices have appeared, saying that Ukraine should give in to [Russian President Vladimir] Putin's demands," Duda said, the first foreign leader to address the Ukrainian parliament in person since Russia's Feb. 24 invasion.
"Only Ukraine has the right to decide about its future," he said.
Speaking to the same parliamentary session, Ukrainian President Volodymyr Zelensky renewed a plea for stronger economic sanctions against Moscow.
"Half-measures should not be used when aggression should be stopped," he said.
Shortly after both finished speaking, an air raid siren was heard in the capital, a reminder that the war raged on even if its front lines are now hundreds of kilometres away.
Zelensky said at a news conference with Duda that 50 to 100 Ukrainians are dying every day on the war's eastern front in what appeared to be a reference to military casualties.
Russia is waging a major offensive in Luhansk, one of two provinces in Donbas, after ending weeks of resistance by the last Ukrainian fighters in the strategic southeastern port city of Mariupol.
The heaviest fighting focused around the twin cities of Sievierodonetsk and Lysychansk, Interior Ministry adviser Vadym Denysenko told Ukrainian television on Sunday.
The cities form the eastern part of a Ukrainian-held pocket that Russia has been trying to overrun since mid-April after failing to capture Kyiv and shifting its focus to the east and south of the country.
Russia's Defence Ministry said on Sunday its forces pummelled Ukrainian command centres, troops and ammunition depots in Donbas and the Mykolaiv region in the south with airstrikes and artillery.
Reuters was unable to independently verify those battlefield reports.
Russian-backed separatists already controlled parts of Luhansk and neighbouring Donetsk before the invasion, but Moscow wants to seize the remaining Ukrainian-held territory in the region.
An explosion severely injured a Russian-appointed mayor in the town of Enerhodar, Russia's RIA news agency reported. Reuters could not immediately establish what caused the explosion.
The end of fighting in Mariupol, the biggest city Russia has captured, gave 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 have surrendered, the Russian Defence Ministry said on Friday.
While Ukraine has not confirmed a full withdrawal, the commander of the Azov Regiment — one of the units in the factory — said in a video that Ukraine's military command had ordered the troops there to stand down in order to preserve their lives. The regiment is a far-right armed group that was folded into Ukraine's National Guard after Russia's first invasion in 2014.
Full control of Mariupol gives Russia command of a land route linking the Crimean Peninsula, which Moscow seized in 2014, with mainland Russia and parts of Eastern Ukraine held by pro-Russia separatists.
Putin calls the invasion a "special military operation" to disarm Ukraine and rid it of radical anti-Russian nationalists. Ukraine and its allies have dismissed that as a baseless pretext for the war, which has killed thousands of people in Ukraine, displaced millions and shattered cities.