Russian troops walk in a destroyed part of the steel works in Mariupol. AP

He said people are being killed or injured. Homes, civilian infrastructure and residential buildings have been damaged or destroyed.

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Russia intensifies Donbas offensive as war enters fourth month

Leaders navigating dark hour after Ukraine invasion: Biden

In the government-controlled part of Luhansk, local authorities informed the UN that a bridge leading to the administrative center of the region — Sievierodonetsk — was destroyed on May 21. He said that left the partially encircled city reachable by only one road.

Vehicles are on fire at an oil depot after missiles struck in Makiivka, Donetsk, Ukraine. AP

While some people managed to leave Sievierodonetsk over the weekend, Dujarric said local authorities estimate that thousands of civilians remain in the war-affected city and require urgent support.

UN humanitarian staff also said that shelling and airstrikes were reported in other areas of Ukraine, including in northern, central and southern parts, claiming civilian lives and damaging civilian infrastructure.

A Ukrainian official says Russian forces are stepping up their bombardment of the Donbas area.

Smoke rises from the Metallurgical Combine Azovstal in Mariupol during shelling. AP

Pavlo Kyrylenko told The Associated Press on Monday in Kramatorsk that heavy fighting was continuing near the region of Luhansk and that the front line was under continuous shelling.

Kramatorsk and neighboring Sloviansk are the largest cities in the parts of Donetsk region not held by Russian forces currently. The Donbas consists of the Donetsk and Luhansk regions.

Kyrylenko told The Associated Press in Kramatorsk that the "situation is difficult. The front line is under shelling at all times.”

The vast majority of the population has already been evacuated, he said. Of more than 1.6 million people who lived in the region before the Feb. 24 Russian invasion, there are "not more than 320,000 people” left.

Associated Press