Russian forces pounded dozens of targets in eastern Ukraine with airstrikes and artillery fire as Ukrainians were digging in to defend the eastern city of Sievierodonetsk, as it endured heavy bombardment from Russian forces attempting to take the industrial area of Donbass.

In eastern Ukraine, the Russian air force hit four command centres, a communications point, an anti-aircraft missile system and 87 areas where troops and Ukrainian military equipment amassed as well as seven ammunition stores, the Russian Defense Ministry said.

Russian artillery hit 73 command points, 578 areas where troops and Ukrainian military equipment amassed, as well as 37 artillery and mortar units in firing positions, the defence ministry said. Russia said it also shot down three Ukrainian Su-25 jets.

Sea-launched long range missiles hit Ukrainian weapons at the Malin railway station in western Ukraine that were being transported to the east, the defense ministry said.

In Tokyo on Monday, U.S. President Joe Biden and Japanese Prime Minister Fumio Kishida joined in condemning Moscow’s invasion of Ukraine. Earlier on his trip to Asia, Biden signed legislation granting Ukraine $40 billion more in U.S. support for its defense against the Russian attack.

Shelling increased in ferocity as Russian and Ukrainian forces battled along a 551-kilometer (342-mile) wedge of Ukraine’s eastern industrial heartland, driving residents to flee.

Russian forces tried to storm Sievierodonetsk, but were unsuccessful and retreated, Ukrainian President Volodymyr Zelenskyy's office said.

Polish President Andrzej Duda traveled to Kyiv on Sunday to support Ukraine’s European Union aspirations and addressed Ukraine's parliament, receiving a standing ovation when he thanked the lawmakers for letting him speak where "the heart of a free, independent and democratic Ukraine beats."

Ukraine does not have to yield to pressure from Russia and from elsewhere in Europe, Duda 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."

Poland has become an important ally of Ukraine, welcoming millions of Ukrainian refugees and becoming a gateway for Western humanitarian aid and weapons, and a transit point for some foreign fighters who have volunteered to fight Russian forces.

Zelenskyy called Duda's visit, his second since April, "a historic opportunity not to lose such strong relations, built through blood, through Russian aggression. All this not to lose our state, not to lose our people."

On the battlefield, grinding, town-by-town fighting continued as Russian troops try to expand the territory that Moscow-backed separatists have held since 2014 in the Donbass, which includes Luhansk and Donetsk.

Sievierodonetsk is the main city under Ukrainian control in Luhansk province, whose governor Serhii Haidai accused the Russians of "simply intentionally trying to destroy the city ... engaging in a scorched-earth approach."

Haidai said the Russians had occupied several towns and cities in Luhansk after indiscriminate, 24-hour shelling and were concentrating forces and weaponry there, bringing in troops from Kharkiv to the northwest, Mariupol to the south, and from inside Russia.

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

The Ukrainian military said Russian forces were unsuccessful in their attack on Oleksandrivka, a village outside the city.

Ukraine’s parliament voted Sunday to extend martial law and mobilize its armed forces for a third time, until Aug. 23. Ukrainian officials have said little since the war began about the extent of their country’s casualties, but Zelenskyy said 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 preparing to resume its offensive on Slovyansk, a city in Donetsk province that saw fierce fighting last month after Moscow’s troops retreated from Kyiv.

Fully captured Mariupol

Despite pouring its forces into those areas and launching massive artillery bombardments, it has made only small territorial gains there, meanwhile continuing to lose territory in a Ukrainian counterattack further north around Kharkiv. But the full capture of Mariupol last week gives Russia its biggest victory for months. Its forces now control a largely unbroken swathe of the east and south, freeing up more troops to join the main Donbass fight.

The conflict is not confined to Ukraine’s east. Powerful explosions were heard early Monday in Korosten, about 160 kilometers (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.

Meanwhile, a captured Russian soldier was sentenced to life in prison on Monday for killing a Ukrainian civilian, sealing the first guilty conviction for war crimes since Russia's invasion three months ago. Sgt. Vadim Shishimarin, 21, pleaded guilty to shooting a Ukrainian civilian in the head in a village in the northeastern Sumy region in the early days of the war.

Ukrainian Prosecutor General Iryna Venediktova 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."