Ten weeks into a devastating war, Ukrainian and Russian forces are fighting village by village, as Moscow struggles to gain momentum in the eastern industrial heartland of the Donbas. Russia switched its focus to that region — where Moscow-backed separatists have fought Ukrainian forces for years — after a stiffer than expected resistance bogged its troops down and thwarted its initial goal of overrunning the capital.

In addition to heavy shelling of the Donbas, Russian forces also kept up their bombardment of railroad stations and other supply-line targets across the country — part of an effort to disrupt the supply of Western arms, which have been critical to Ukraine's defense.

Ukrainian forces said Thursday they made some gains on the border of the southern regions of Kherson and Mykolaiv and repelled 11 Russian attacks in the Donetsk and Luhansk regions that make up the Donbas.?

Five people were killed and at least 25 more wounded in shelling of cities in the Donbas over the past 24 hours, Ukrainian officials said. The attacks damaged houses and a school as well.

Air raid sirens sounded in cities across the country on Wednesday night, while Russian 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 — following several earlier attacks on railway stations across the country. The sirens sounded anew early Thursday in the western city of Lviv, which has been a gateway for western arms and served as a relative safe haven for people fleeing fighting farther east.

An assessment by the Washington-based Institute for the Study of War said Russian forces were struggling to gain traction.

“Ukrainian defenses have largely stalled Russian advances in eastern Ukraine,” it said late Wednesday.?

“Russian forces intensified airstrikes against transportation infrastructure in western Ukraine (on Wednesday) but remain unable to interdict Western aid shipments to Ukraine,” it added.

In the most searing example of how Ukrainian forces have slowed Russia's progress, Ukrainian fighters held out at the sprawling Azovstal steel mill in Mariupol — the last pocket of resistance in a city that is otherwise controlled by Moscow's forces.?

“With the support of aircraft, the enemy resumed the offensive in order to take control of the plant,” the Ukrainian military's General Staff said Thursday. A stream of black smoke rose above the plant on Wednesday.

The Ukrainians said Russian forces have pushed into the plant's perimeter and were also bombing it from above. The Kremlin denied that there was any ground assault.

Denys Prokopenko, commander of the Ukrainian Azov regiment that's defending the plant, said in a video posted Wednesday that the incursions continued for a second day, “and there are heavy, bloody battles.”

Mariupol'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 the Donbas.

The city, and the plant in particular, have come to symbolize the misery inflicted by the war. The Russians have pulverized most of Mariupol in a two-month siege that has trapped civilians with little food, water, medicine or heat. Civilians holed up inside the plant have perhaps suffered even more. About 100 of them were evacuated over the weekend — the first time some saw daylight in months.

The Russian government said 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 assurances from the Kremlin have fallen through, with the Ukrainians blaming continued fighting by the Russians.

It is unclear how many Ukrainian fighters are still inside the plant, 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 remain there, the Ukrainian side said this week.

The United Nations announced that more than 300 civilians were evacuated Wednesday from Mariupol and other nearby communities. The evacuees arrived in Zaporizhzhia, about 140 miles (230 kilometers) to the northwest, where they were receiving humanitarian assistance.

“Many came with nothing but the clothes they were wearing, and we will now support them during this difficult time, including with much-needed psychological support,” said Osnat Lubrani, the U.N. humanitarian coordinator for Ukraine.

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 British Defense Ministry said Thursday that it does not anticipate that the drills currently posed a threat to Ukraine, but that Moscow will likely use them "to fix Ukrainian forces in the north, preventing them from being committed to the battle for the Donbas.”

In addition to supplying weapons to Ukraine, Europe and the U.S. have sought to punish Moscow with sanctions, taking aim at the country's vital energy sector. The European Union's top official called on the 27-nation bloc on Wednesday to ban Russian oil imports, ?a crucial source of revenue.

The proposal needs unanimous approval from EU countries and is likely to be debated fiercely. Hungary and Slovakia have already said they won't take part in any oil sanctions, but they could be granted an exemption.

The EU is also talking about a possible embargo on Russian natural gas. The bloc has already approved a cutoff of coal imports.?