(Updates with Russian defence ministry statement)
* Ukraine rules out ceasefire, concessions
* Russia launches assault in Luhansk, Mykolayiv
* Ukraine must decide own future, Polish president says
By Natalia Zinets and Conor Humphries
KYIV, May 22 (Reuters) - Ukraine ruled out a ceasefire or any territorial concessions to Moscow while Russia stepped up its attack in the eastern Donbas region and stopped sending gas to Finland in its latest response to Western sanctions and its deepening international isolation.
Polish President Andrzej Duda told lawmakers in Kyiv that the international community had to demand Russia's complete withdrawal from Ukrainian territory and that sacrificing even an inch of it would be a "huge blow" to the entire West.
"Worrying voices have appeared, saying that Ukraine should give in to (President Vladimir) Putin's demands," Duda said, the first foreign leader to address Ukrainian lawmakers in person since Russia's Feb. 24 invasion.
"Only Ukraine has the right to decide about its future." Shortly after he finished speaking, an air raid siren was heard in central Kyiv, a reminder that the nation remained at war even as the front lines have shifted hundreds kilometres to the south and east.
After ending weeks of resistance by the last Ukrainian fighters in the strategic southeastern port of Mariupol, Russia is waging a major offensive in Luhansk, one of two provinces in Donbas.
Russian-backed separatists already controlled parts of Luhansk and the neighbouring Donetsk province before the invasion, but Moscow wants to seize the remaining Ukrainian-held territory in the region.
The Russian defence ministry said on Sunday its forces pounded Ukrainian command centres, troops and ammunition depots in Donbas and the Mykolayiv region in the south with air strikes and artillery.
Ukraine's general staff reported continued heavy Russian shelling of twin cities Sievierodonetsk and Lysychansk in the Luhansk region.
The cities, separated by 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 and shifting its focus to the east and south of the country.
Reuters was unable to independently verify those battlefield reports.
Ukraine's lead negotiator, speaking to Reuters on Saturday, ruled out a ceasefire that would involve Russian forces remaining in occupied areas or any deal with Moscow that involved ceding territory. Making concessions would backfire because Russia would hit back harder after any break in fighting, President Volodymyr Zelenskiy's adviser Mykhailo Podolyak said.