Civilians, including women and children, will need to be dug out from bunkers under a steelworks that is the last holdout of resistance in Ukraine's Mariupol, President Volodymyr Zelenskyy said Thursday, as Russia moved to obstruct the flow of Western weapons to Ukraine by bombing rail stations and other supply line targets across the country.

After failing to capture the capital Kyiv in the early weeks of a war that has killed thousands and flattened cities, Russia has accelerated attacks on Ukraine's east and south.

Pavlo Kyrylenko, governor of the eastern region of Donetsk, said at least 25 civilians were wounded as Russian forces shelled Kramatorsk, a town some 180 kilometers (112 miles) west of Luhansk.

The United Nations and Red Cross evacuated hundreds of people from Mariupol and other areas this week. But some 200 civilians, as well as Ukrainian fighters, are still holed up in a network of underground bunkers in the Azovstal plant, Ukrainian officials say.

Russia vowed to pause military activity at Azovstal during the day Thursday and the following two days to allow civilians to get out. In an early-morning address, Zelenskyy said Ukraine stood ready to ensure a cease-fire in Mariupol.

"It will take time simply to lift people out of those basements, out of those underground shelters. In the present conditions, we can not use heavy equipment to clear the rubble away. It all has to be done by hand," Zelenskyy said.

Ukrainian fighters inside Azovstal are fighting "difficult, bloody battles" against Russian troops, Denis Prokopenko, a commander with Ukraine's Azov regiment, said late on Wednesday. A Ukrainian parliamentarian said Russian forces were inside the plant.

Over 300 civilians were evacuated on Wednesday from Mariupol and other areas in southern Ukraine as part of a joint U.N.-Red Cross operation, U.N. humanitarian coordinator for Ukraine Osnat Lubrani said.

Combat rages at steel mill

Heavy fighting also raged at the Azovstal steel mill in Mariupol, the last stronghold of Ukrainian resistance in the ruined southern port city, the Ukrainian military reported. A Russian official earlier denied that troops were storming the plant, but the commander of the main Ukrainian unit inside said Russian soldiers had pushed into the mill’s territory.

"With the support of aircraft, the enemy resumed the offensive in order to take control of the plant," the General Staff in Kyiv said, adding that the Russians were "trying to destroy Ukrainian units."

To the west of Mariupol, Ukrainian forces made some gains on the border of the southern regions of Kherson and Mykolaiv, where Russian troops were reportedly trying to launch a counteroffensive, and repelled 11 Russian attacks in the Donetsk and Luhansk regions, the military said.

The Russian military said it used sea- and air-launched missiles to destroy electric power facilities at five railway stations across Ukraine. Artillery and aircraft also struck troop strongholds and fuel and ammunition depots.

Ukrainian Foreign Minister Dmytro Kuleba accused Russia of "resorting to the missile terrorism tactics in order to spread fear across Ukraine."

Air raid sirens sounded in cities across the country on Wednesday night, and attacks were reported near Kyiv, the capital; in Cherkasy and Dnipro in central Ukraine; and in Zaporizhzhia in the southeast. In Dnipro, authorities said a rail facility was hit. Videos on social media suggested a bridge there was attacked.

Responding to the strikes in his nightly video address, Zelenskyy said: "All of these crimes will be answered, legally and quite practically – on the battlefield."

The flurry of attacks comes as Russia prepares to celebrate Victory Day on May 9, marking the Soviet Union’s defeat of Nazi Germany. The world is watching whether Russian President Vladimir Putin will use the occasion to declare a victory in Ukraine or expand what he calls the "special military operation."

A declaration of all-out war would allow Putin to introduce martial law and mobilize reservists to make up for significant troop losses. Kremlin spokesperson Dmitry Peskov dismissed the speculation as "nonsense."

Meanwhile, Belarus, which Russia used as a staging ground for its invasion, announced the start of military exercises Wednesday. A top Ukrainian official said the country will be ready to act if Belarus joins the fighting.

The attacks on rail infrastructure were meant to disrupt the delivery of Western weapons, Russian Defense Ministry spokesperson Maj. Gen. Igor Konashenkov said. Defense Minister Sergei Shoigu complained that the West is "stuffing Ukraine with weapons."

A senior U.S. defense official, speaking on condition of anonymity to discuss the Pentagon's assessment, said that while the Russians have tried to hit critical infrastructure around the western city of Lviv, specifically targeting railroads, there has been "no appreciable impact" on Ukraine’s effort to resupply its forces. Lviv, close to the Polish border, has been a major gateway for NATO-supplied weapons.

Weaponry pouring into Ukraine helped its forces thwart Russia’s initial drive to seize Kyiv and seem certain to play a central role in the growing battle for Donbass, the eastern industrial region that Moscow now says is its main objective.

Ukraine has urged the West to ramp up the supply of weapons ahead of that potentially decisive clash.

In addition to supplying weapons to Ukraine, Europe and the U.S. have sought to punish Moscow with sanctions. The EU's top official called on the 27-nation bloc on Wednesday to ban Russian oil imports, a crucial source of revenue.

In Mariupol, Mayor Vadym Boychenko said that Russian forces were targeting the already shattered Azovstal plant with heavy artillery, tanks, aircraft, warships and "heavy bombs that pierce concrete 3 to 5 meters thick."

Ukrainian fighters said Tuesday that Russian forces had begun storming the plant. But the Kremlin denied it. "There is no assault," Peskov said.

Denys Prokopenko, commander of the Ukrainian Azov regiment that's defending the plant, said Russian forces got into the plant's territory.

Prokopenko said in a video that the incursions continued for a second day, "and there are heavy, bloody battles."

"The situation is extremely difficult, but in spite of everything, we continue to carry out the order to hold the defense," he added.

Meanwhile, the U.N. announced that more than 300 civilians were evacuated Wednesday from Mariupol and other nearby communities. The evacuees arrived in Zaporizhzhia, about 140 miles to the northwest, where they were receiving humanitarian assistance.

Over the weekend, more than 100 people – including women, the elderly and 17 children – were evacuated from the plant during a cease-fire in an operation overseen by the U.N. and the Red Cross. But the attacks on the plant soon resumed.

The Russian government said on the Telegram messaging app that it would open another evacuation corridor from the plant during certain hours on Thursday through Saturday. But there was no immediate confirmation of those arrangements from other parties, and many previous such assurances from the Kremlin have fallen through, with the Ukrainians blaming the continued fighting by the Russians.

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, the Ukrainian side said.

Mariupol, and the plant, in particular, have come to symbolize the misery inflicted by the war. The Russians have pulverized most of the city in a two-month siege that has trapped civilians with little food, water, medicine or heat.

The city's 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 to fight elsewhere in Donbass.