Russian forces opened artillery fire on towns in eastern Ukraine, killing and wounding dozens of civilians, and began storming the steel mill in Mariupol as Ukrainian officials hold out hope for more evacuations from the site, where scores of civilians reached relative safety after enduring weeks of shelling that targeted the city's last pocket of resistance.

The governor of the eastern Donetsk region said Russian attacks left 21 dead on Tuesday, the highest number of known fatalities since April 8, when a missile attack on the railway station in Kramatorsk killed at least 59 people.

Thanks to the evacuation effort over the weekend, at least 101 people – including women, the elderly and 17 children, the youngest 6 months old – emerged from the bunkers under Mariupol's Azovstal steelworks to "see the daylight after two months," said Osnat Lubrani, the U.N. humanitarian coordinator for Ukraine.

Ukraine’s Deputy Prime Minister Iryna Vereshchuk said authorities on Wednesday plan to continue efforts to evacuate civilians from Mariupol and nearby areas if the security situation allows it. Lubrani also expressed hope for further evacuations but said none had been worked out.

Evacuees, a few of whom were in tears, made their way from the buses into a tent offering food, diapers and connections to the outside world. Some of the evacuees browsed racks of donated clothing, including new underwear.

The news for those left behind was more grim. Ukrainian commanders said Russian forces backed by tanks began storming the sprawling plant, which includes a maze of tunnels and bunkers spread out over 11 square kilometers (4 square miles).

It was unclear how many Ukrainian fighters were still inside, but the Russians put the number at about 2,000 in recent weeks, and 500 were reported to be wounded. A few hundred civilians also remained there, Vereshchuk said.

"We’ll do everything that’s possible to repel the assault, but we’re calling for urgent measures to evacuate the civilians that remain inside the plant and to bring them out safely," Sviatoslav Palamar, deputy commander of Ukraine’s Azov Regiment, said on the messaging app Telegram.

He added that throughout the night, the plant was hit with naval artillery fire and airstrikes. Two civilian women were killed and 10 civilians wounded, he said.

Russia warns NATO

Russia had fired two Kalibr cruise missiles at Ukrainian targets from a submarine in the Black Sea and reiterated a warning that it would seek to hit shipments of NATO weapons to Ukraine.

The Russian Defense Ministry published video footage of the cruise missiles being launched from the Black Sea, and said they had hit unspecified ground targets in Ukraine.

Russia previously said it had mounted similar strikes from a submarine on April 29.

Earlier on Wednesday, Defense Minister Sergei Shoigu reiterated a warning that Russia would seek to destroy convoys of arms shipments to Ukraine from Western countries, which in recent weeks have stepped up these supplies.

"The U.S. and its NATO allies are continuing to pump weapons into Ukraine," Shoigu told a conference of Defense Ministry officials on day 70 of what Russia calls its special military operation in Ukraine.

"We view any transport of the North Atlantic Alliance arriving on the territory of the country with weapons or materials destined to the Ukrainian army as a target to be destroyed," he added.

Russia's Defense Ministry said earlier that it had disabled six railway stations in Ukraine used to supply Ukrainian forces with Western-made weapons in the country's east by bombing their power supplies.

In his nightly video address, Ukrainian President Volodymyr Zelenskyy said that by storming the steel mill, Russian forces violated agreements for safe evacuations. He said the prior evacuations are "not a victory yet, but it’s already a result. I believe there’s still a chance to save other people."

Among those killed in fresh attacks in Donetsk on Tuesday were 10 people at a chemical plant in the city of Avdiivka, Donetsk Governor Pavlo Kyrylenko said.

"The Russians knew exactly where to aim – the workers just finished their shift and were waiting for a bus at a bus stop to take them home," he wrote in a Telegram post. "Another cynical crime by Russians on our land," he added.

Another two civilians were killed and two wounded in overnight shelling in the neighboring Luhansk region, Governor Serhiy Haidai said, adding that Russian attacks were intensifying.

Just to the north, near the strategic junction city of Izyum, Russia has deployed 22 battalion tactical groups in its attempt to advance along the northern axis of the Donbass, the British Defense Ministry reported Wednesday. Each unit typically numbers about 800 soldiers.

Despite appearing to struggle in breaking through Ukrainian defenses and build momentum, Russia likely intends to proceed beyond Izyum to capture the cities of Kramatorsk and Severodonetsk, as they try to cut Ukrainian forces off in the region, according to the British assessment. However, Moscow’s push has been slow as Ukrainian fighters dig in and use long-range weapons, like howitzers, to target the Russians.

The U.S. believes Ukrainians in recent days have pushed Russian forces about 40 kilometers (25 miles) east of Kharkiv, which lies outside Donbass but is key to the offensive there. Extending the distance of the front line makes it more difficult for Russia to target the city with artillery fire.

Ukraine’s eastern industrial heartland of Donbass, which includes Donetsk and Luhansk, remains Moscow's stated objective after failing to take Kyiv in the early weeks of the war.

Explosions were also heard in Lviv, in western Ukraine, near the Polish border. The strikes damaged three power substations, knocking out electricity in parts of the city, disrupting the water supply and wounding two people, the mayor said. Lviv has been a gateway for NATO-supplied weapons and a haven for those fleeing the fighting in the east.

Russian Defense Ministry spokesperson Maj. Gen. Igor Konashenkov said Russian aircraft and artillery hit hundreds of targets in the past day, including troop strongholds, command posts, artillery positions, fuel and ammunition depots and radar equipment.

Ukrainian authorities said the Russians also attacked at least a half-dozen railroad stations around the country.

The assault on the Azovstal steelworks began almost two weeks after Russian President Vladimir Putin ordered his military not to storm the plant to finish off the defenders but to block it off. The first – and so far only – civilians to be evacuated from the shattered plant got out during a brief cease-fire in an operation overseen by the U.N. and Red Cross.

In addition to the 101 people evacuated from the steelworks, 58 joined the convoy in a town on the outskirts of Mariupol, Lubrani said. About 30 people who left the plant decided to stay behind in Mariupol to try to find out whether their loved ones were alive, Lubrani said. A total of 127 evacuees arrived in Zaporizhzhia, she said.

The Russian military said earlier that some of the evacuees chose to stay in areas held by pro-Moscow separatists.

Mariupol has come to symbolize the human misery inflicted by the war. The Russians’ two-month siege of the strategic southern port has trapped civilians with little or no food, water, medicine or heat, as Moscow’s forces pounded the city into rubble. The plant in particular has transfixed the outside world.

Mariupol's complete fall would deprive Ukraine of a vital port, allow Russia to establish a land corridor to the Crimean Peninsula, which it seized from Ukraine in 2014, and free up troops for fighting elsewhere in Donbass.