The government in Warsaw is a strong supporter of Ukraine's bid to join the EU, which Zelenskyy says will determine Ukraine's future.
Russia pressed its offensive in Ukraine's eastern Donbas region on Sunday as Ukrainian President Volodymyr Zelenskyy said the outcome of the gruelling conflict would determine whether his country's fate lies with the West or under Moscow's domination.
After declaring its total control of Azovstal, a sprawling steel mill complex that was the last defence holdout in the port city of Mariupol, the Russian military launched strikes in Ukraine's industrial heartland, seeking to expand the territory Moscow-backed separatists have held since 2014.
In a Saturday night video address to the nation, Zelenskyy described the situation in the Donbas as "really hard", but "the fact that we are able to say this on the 87th day of a full-scale war against Russia is good news."
"Every day that our defenders take away from these offensive plans of Russia, disrupting them, is a concrete contribution in the approach of the main day. The desired day that we are all looking forward to and fighting for: Victory Day," Zelenskyy said.
Duda speaks to Rada
Zelenskyy's remarks came as Poland's president prepared to meet with him to support Ukraine's goal of becoming a candidate for EU membership, an issue set to be decided at the bloc's summit in late June.
As the West rallied behind Ukraine, Polish President Andrzej Duda made an unannounced visit to Kyiv and on Sunday became the first foreign leader to address the Ukrainian parliament, the Rada, since the start of the war.
In a rousing speech in front of the Ukrainian MPs, who interrupted him several times with standing ovations, Duda said that the world cannot go back to "business as usual" with Moscow after the discovery of massacres of civilians in places like Bucha and Mariupol.
An honest world cannot return to the agenda by forgetting the crimes, the aggression, the trampled fundamental rights," the Polish president added in the presence of his Ukrainian counterpart.
Poland, which has welcomed millions of Ukrainian refugees since Russia invaded its neighbour, has become a significant gateway for Western humanitarian aid and weapons going into Ukraine. The government in Warsaw also strongly supports Ukraine's desire to join the EU.
Affirming that he would not relax his efforts "until Ukraine is a member of the EU," Duda said he regretted that "voices have been raised recently in Europe asking that Ukraine accept certain demands from (Russian President Vladimir) Putin".
"If for their own peace of mind, economic interests or political ambitions", Western countries were to "sacrifice Ukraine -- even one centimetre of its territory or a piece of its sovereignty -- it will be a great blow to the Ukrainian people, but also to the entire Western community", Mr Duda warned.
"Only Ukraine has the right to decide its future," Duda said. "There can be no negotiations or decisions taken behind Ukraine's back. Nothing about you without you. Absolutely not. This is an inflexible rule. It must be adhered to."
"I thank you for defending Europe against the invasion of the barbarians and the new Russian imperialism," Duda emphasised, saying that "Russia had not achieved any of its strategic objectives" in Ukraine.
The Ukrainian President, for his part, hailed what he deemed "a historic moment ... a historic union between the Ukrainian and Polish peoples."
"Poland plays an important role in our future, in our common future in the European Union," he said, and if Ukraine were to become a member of the EU, "it will be thanks to Poland".
Zelenskyy stressed earlier on Saturday that Brussels should consider Ukraine's desire to join the 27-nation bloc as soon as possible within the context of Russia's invasion.
"I want to emphasize that our European integration path is not just about politics," Zelenskyy said. "It's about quality of life. And about the fact that Ukrainians perceive the values of life in the same way as the vast majority of Europeans."
Fighting in the Donbas escalates
Russia appeared to have made slow grinding moves forward in the Donbas in recent days. It intensified efforts to capture Sievierodonetsk, the main city under Ukrainian control in Luhansk province, which, together with Donetsk province, makes up the Donbas.
Luhansk Governor Serhiy Haidai said the only functioning hospital in the city has just three doctors and enough supplies for 10 days.
On Sunday, the UK Ministry of Defence said Russia's only operational company of BMP-T Terminator tank support vehicles, designed to protect main battle tanks, "has likely been deployed to the Sievierodonetsk axis of the Donbas offensive."
It said, however, that with a maximum of 10 of the vehicles deployed, "they are unlikely to have a significant impact on the campaign."
In a general staff morning report, Russia also said that it was preparing to resume its offensive toward Slovyansk, a city in Donetsk province critical to Russia's objective of capturing all of eastern Ukraine and saw fierce fighting last month after Moscow's troops backed off from Kyiv.
Russian shelling on Saturday killed seven civilians and injured 10 more elsewhere in Donetsk province, the regional governor said.
A monastery in the village of Bohorodichne was evacuated after being hit by a Russian airstrike, the regional police said Saturday. About 100 monks, nuns and children had been seeking safe shelter in the church's basement, and no one was hurt, the police said in a Facebook post.
Zelenskyy emphasized that the Donbas remains part of Ukraine, and his forces were fighting to liberate it.
Fall of Azovstal a victory in Putin's eyes
With Russia claiming to have taken prisoner nearly 2,500 Ukrainian fighters from the besieged Mariupol steel plant, concerns grew about their fate and the future faced by the city's remaining residents, now in ruins with more than 20,000 residents feared dead.
The Russian Defense Ministry released a video of Ukrainian soldiers being detained after announcing that its forces had removed the last holdouts from the Mariupol plant's extensive underground tunnels. It said a total of 2,439 had surrendered.
Family members of the fighters, who came from various military and law enforcement units, have pleaded for them to be given rights as prisoners of war and eventually returned to Ukraine. Deputy Prime Minister Iryna Vereshchuk said Saturday that Ukraine "will fight for the return" of every one of them.
On Saturday, the Russian Defence Ministry released a video of its troops taking into custody Serhiy Volynskyy, the commander of the Ukrainian Navy's 36th Special Marine Brigade, which was one of the leading forces defending the steel plant.
The Azovstal steel plant became a symbol of Ukrainian tenacity for its weeks-long persistence in relenting to attempts by the Russians to seize it. Its fall gave Russian President Vladimir Putin a badly wanted victory in the war he began nearly three months ago.
Denis Pushilin, the pro-Kremlin head of the self-proclaimed Donetsk People's Republic, vowed the Ukrainian fighters from the plant would face tribunals. He said the soldiers included some foreign nationals, though he did not provide details.
A prominent member of Russia's parliament, Leonid Slutsky, said Moscow was studying the possibility of exchanging the Azovstal fighters for Viktor Medvedchuk, a wealthy Ukrainian with close ties to Putin who faces criminal charges in Ukraine, the Russian news agency Interfax reported.
Slutsky later walked back those remarks, saying he agreed with Pushilin that a tribunal should decide their fate.
Southern port faces health catastrophe
The Ukrainian government has not commented on Russia's claim of capturing Azovstal. Ukraine's military had told the fighters their mission was complete, and they could come out. It described their extraction as an evacuation, not a mass surrender.
Mariupol, which is part of the Donbas, was blockaded early in the war and became a frightening example to people elsewhere in the country of the hunger, terror and death they might face if the Russians surrounded their communities.
The mayor of Mariupol warned Saturday that the city is facing a health and sanitation "catastrophe" from mass burials in shallow pits across the ruined city and the breakdown of sewage systems.
Mayor Vadim Boychenko said summer rains threaten to contaminate water sources as he pressed Russian forces to allow residents to leave the city. An estimated 100,000 of the 450,000 people who resided in Mariupol before the war remain.
"In addition to the humanitarian catastrophe created by the occupiers and collaborators, the city is on the verge of an outbreak of infectious diseases," he said on the messaging app Telegram.
With Russia controlling the city, Ukrainian authorities are likely to face delays in documenting evidence of alleged Russian atrocities in Mariupol, including the bombings of a maternity hospital and a theatre where hundreds of civilians had taken cover.
Satellite images in April showed what appeared to be mass graves just outside Mariupol, where local officials accused Russia of concealing the slaughter by burying up to 9,000 civilians.