KYIV, Ukraine (AP) — Russia pressed its offensive in eastern Ukraine on Sunday as Poland’s president traveled to Kyiv to support the country’s European Union aspirations, becoming the first foreign leader to address the Ukrainian parliament since the start of the war.

Lawmakers gave a standing ovation to President Andrzej Duda, who thanked them for the honor of speaking where “the heart of a free, independent and democratic Ukraine beats.” Duda said that to end the conflict, Ukraine did not need to submit to conditions given by Russian President Vladimir Putin.

“Unfortunately, in Europe there have also been disturbing voices in recent times demanding that Ukraine yield to Putin’s demands,” he said. “I want to say clearly: Only Ukraine has the right to decide about its future. Only Ukraine has the right to decide for itself.”

Duda’s visit, his second to Kyiv since April, came as Russian and Ukrainian forces battled along a 342-mile wedge of the country’s eastern industrial heartland.

After declaring full control of a sprawling seaside steel plant that was the last defensive holdout in the port city of Mariupol, Russia launched artillery and missile attacks to expand the territory that Moscow-backed separatists have held since 2014 in the region known as the Donbas.

To bolster its defenses, Ukraine’s parliament voted Sunday to extend martial law and the mobilization of armed forces for a third time, until Aug. 23.

Ukrainian President Volodymyr Zelenskyy has stressed that the 27-member EU should expedite his country’s request to join the bloc. Ukraine’s potential candidacy is set to be discussed at a Brussels summit in late June.

France’s European Affairs minister Clement Beaune on Sunday told Radio J it would be a “long time” before Ukraine gains EU membership, perhaps up to two decades.

“We have to be honest,” he said. “If you say Ukraine is going to join the EU in six months, or a year or two, you’re lying.”

But Poland is ramping up efforts to win over EU members who are more hesitant about accepting Ukraine into the bloc. Zelenskyy said Duda’s visit represented a “historic union” between Ukraine, which declared independence from the Soviet Union in 1991, and Poland, which ended communist rule two years earlier.

“This is really a historic opportunity not to lose such strong relations, built through blood, through Russian aggression,” Zelenskyy said. “All this not to lose our state, not to lose our people.”

Poland has welcomed millions of Ukrainian refugees and become a gateway for Western humanitarian aid and weapons into Ukraine. It is also a transit point for some foreign fighters who have volunteered to fight the Russian forces.

Duda credited the U.S. and President Joe Biden for unifying the West in supporting Ukraine and imposing sanctions against Moscow.

“Kyiv is the place from which one clearly sees that we need more America in Europe, both in the military and in this economic dimension,” said Duda, a right-wing populist leader who clearly preferred former President Donald Trump over Biden in the 2020 election.

On the battlefield, Russia appeared to have made slow, grinding advances 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. The Ukrainian military said Sunday that Russian forces had mounted an unsuccessful attack on Oleksandrivka, a village outside of Sievierodonetsk.

Sievierodonetsk came under heavy shelling, and Luhansk Gov. Serhii Haidai said the Russians were “simply intentionally trying to destroy the city... engaging in a scorched-earth approach.”

Haidai said Moscow was concentrating forces and weaponry there to try to win control of Luhansk, bringing in forces from Kharkiv to the northwest, Mariupol to the south, and from inside Russia.

The sole working hospital in the city has only three doctors and supplies for 10 days, he said.

Ukrainian officials have said little since the war began about the extent of their country’s casualties, but Zelenskyy said at a news conference Sunday that 50 to 100 Ukrainian fighters were being killed, apparently each day, in the east.

In a general staff morning report, Russia said it was also preparing to resume its offensive on Slovyansk, a city in Donetsk province that saw fierce fighting last month after Moscow’s troops backed away from Kyiv.

The conflict was not confined to Ukraine’s east. Powerful explosions were heard early Monday, for example, in Korosten, about 100 miles west of Kyiv, the town’s deputy mayor said. It was the third straight day of apparent attacks in the Zhytomyr District, Ukrainian news agencies reported.

In Enerhodar, a Russian-held city 174 miles northwest of Mariupol, an explosion Sunday injured the Moscow-appointed mayor at his residence, Ukrainian and Russian news agencies reported. Ukraine’s Unian news agency said a bomb planted by “local partisans” wounded 48-year-old Andrei Shevchuk, who lives near the Zaporizhzhia Nuclear Power Plant, Europe’s largest.

With Russia claiming to have taken prisoner nearly 2,500 Ukrainian fighters from the Mariupol steel plant, concerns grew about their fate and that of the remaining residents of the city, now in ruins with more than 20,000 feared dead.

Relatives of the fighters have pleaded for them to be given rights as prisoners of war and eventually returned to Ukraine. Deputy Prime Minister Iryna Vereshchuk said Ukraine “will fight for the return” of every one of them.

Denis Pushilin, the pro-Kremlin head of the self-proclaimed Donetsk People’s Republic, vowed that the Ukrainian fighters from the plant would face tribunals.

The complete seizure of the Azovstal steel plant, a symbol of Ukrainian tenacity, gave Putin a badly wanted victory in the war he began nearly three months ago, on Feb. 24. 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 Mayor Vadim Boychenko warned that the city faces a health and sanitation “catastrophe” from mass burials in shallow pits and the breakdown of sewage systems. An estimated 100,000 of the 450,000 people who lived in Mariupol before the war remain.

Ukrainian authorities have alleged Russian atrocities there, including the bombings of a maternity hospital and a theater where hundreds of civilians had taken cover.

Meanwhile, a Ukrainian court was expected to reach a verdict Monday for a Russian soldier who was the first to go on trial for an alleged war crime. The 21-year-old sergeant, who has admitted to shooting a Ukrainian man in the head in a village in the northeastern Sumy region on Feb. 28, could get life in prison if convicted.

Ukrainian Prosecutor General Iryna Venediktova has said her office was prosecuting war crimes cases against 41 Russian soldiers for offenses that included bombing civilian infrastructure, killing civilians, rape and looting.

In other developments, Ukraine’s first lady, Olena Zelenska, gave a rare interview to national broadcaster ICTV alongside her husband and said she has hardly seen him since the war began.

“Our family, like all Ukrainian families, is now separated,” she said, adding that she speaks to him mostly by phone.

“Unfortunately, we cannot sit together, have dinner with the whole family, talk about everything,” she said.

Zelenskyy called the interview itself “a date on air,” and the couple, who have two children, joked in front of the journalists.

“We are joking, but we are really waiting, like everyone else, to be reunited, like all families in Ukraine who are separated now, waiting for their relatives and friends who want to be together again,” he said.

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Becatoros reported from Donetsk. Associated Press journalists Yuras Karmanau in Lviv, Andrea Rosa in Kharkiv and other AP staffers around the world contributed.