Kyiv, Ukraine - Volodymyr Zelensky says officials believe that about 2,500 to 3,000 Ukrainian troops have died so far in the war with Russia, which has gone on for more than 50 days.

In a wide-ranging interview with CNN's Jake Tapper, excerpts of which were released on Friday, Zelensky added that about 10,000 Ukrainian troops have been injured, also saying and that it's "hard to say how many will survive."

Zelensky compared those numbers with what he said were 19,000 to 20,000 Russian fatalities.

Western estimates assume several thousand dead on the Russian side. Moscow recently spoke of around 1,350 soldiers killed in its own ranks.

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More than 2,800 civilians have been successfully evacuated from some of the most fiercely contested cities in eastern Ukraine, according to Ukrainian sources.

Some 2,500 refugees arrived in the southern city of Zaporizhzhya on Friday, including 363 from the devastated port city of Mariupol, Ukrainian Deputy Prime Minister Iryna Vereshchuk announced on Telegram.

Earlier, the Kharkiv Oblast prosecutor's office reported that seven civilians had been killed by Russian soldiers while attempting to use a safe corridor to escape the village of Borova by bus. A further 27 people were reportedly injured in the attack.

The Ukrainian government also alleged that Russia had employed Tu-22M3 supersonic bombers in the conflict, using them to bomb the largely destroyed port city of Mariupol, where fighting continued, despite Russian claims to be in total control, according to Ukrainian Defence Ministry spokesperson Oleksandr Motuzyanyk.

No aid convoys had been allowed into the city of Mariupol since it was surrounded by Russian forces a fortnight ago, World Food Programme (WFP) chief David Beasley said on Friday after a visit to Ukraine. Some 100,000 people are believed to still be in the city, but the last supplies of food and water would soon be exhausted, he added.

Elsewhere in Ukraine, Russian forces continued to bombard the towns of Popasna and Rubizhne in Ukraine's eastern Luhansk region, the Ukrainian military reported.

At least 10 people have been killed in shelling in Kharkiv in eastern Ukraine, the regional state prosecutor's office said on Facebook late Friday. At least 35 people were reportedly injured.

According to Ukrainian sources, the city of Severodonetsk in the Luhansk region has been badly damaged by Russian attacks.

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The head of the city's military administration, Olexandr Strjuk, said the city is about 70% destroyed. The most important roads were also badly damaged and the water supply had also been shut down until repair work could be carried out, Strjuk said on Friday on Ukrainian national television.

Kiev, which is strengthening its troop numbers in the area, expects a larger-scale Russian offensive in eastern Ukraine in the coming days after the Kremlin switched its focus from the capital and areas in the north.

Russian troops previously deployed near the northern city of Chernihiv are now stationed around Severodonetsk in the Luhansk region, the Ukrainian military report said.

If confirmed, this would be the first unit to have been withdrawn and redeployed to eastern Ukraine from north-eastern Ukraine, according to the Washington DC-based Institute for the Study of War (ISW).

"These units likely remain degraded, and Russian forces will face challenges integrating units from several military districts into a cohesive fighting force," ISW said in an assessment.

Russian-backed separatists have already been fighting in Donetsk and Luhansk since 2014, but Moscow is now reinforcing them. Ukraine said it had destroyed several Russian tanks and an artillery system in the area over the past 24 hours.

According to UN figures, more than 4.7 million Ukrainians have left Ukraine since the war began in late February, with another 7 million people being internally displaced by the conflict.