The area, particularly around the town of Bakhmut, is now the main focus of fierce fighting after Ukraine's forces retook Kherson in the south last month.

Russia and its proxies have controlled parts of Donetsk since 2014.

On Tuesday evening, new Russian strikes were reported in the central-eastern Dnipropetrovsk region. One person was injured after an industrial facility was hit in the city of Kryvyi Rih.

This comes after Monday's massive Russian missile strikes, which again targeted critical civilian infrastructure in Ukraine.

The country is now seeing snow and sub-zero temperatures in many regions, and millions are without electricity and running water, raising fears people may die of hypothermia.

Speaking on the Day of the Armed Forces on Tuesday, Zelensky called the fighting around Bakhmut the "most difficult area that protects not only the east but our entire state".

He said he believed Ukraine would take back all the land occupied by Russia, including Crimea, Ukraine's southern peninsula annexed by Russia eight years ago. 

There have been intense battles for many weeks near Bakhmut as Russia attempts to take more territory after a string of battlefield defeats in other parts of Ukraine.

The private military contractor Wagner Group, according to British intelligence, is leading Russia's efforts there.

It's thought both sides have suffered heavy losses in the area. The battle for Bakhmut has been compared to fighting during World War I because of the trenches, charred trees and mud that have become a feature of the landscape.

Zelensky praised troops for their courage and endurance in a video filmed in the city of Sloviansk, about 45km north-west of Bakhmut. Sloviansk was one of the first cities occupied by pro-Russian forces in 2014 but was later recaptured by Ukraine.

Later on Tuesday, Zelensky visited wounded Ukrainian soldiers being treated in a hospital.

Meanwhile, the mayor of the city of Donetsk, which is under Russian control, said Ukrainian shelling had killed six people there on Tuesday.

On Monday, explosions hit two airfields deep inside Russia, which Moscow blamed on Ukrainian drones. 

Another explosion on Tuesday morning close to an airfield in the Kursk region, which borders Ukraine, was also blamed on Kyiv.

Russian President Vladimir Putin later met with top officials to discuss "domestic security"  but no other details were given.

BBC