Ukrainian President Volodymyr Zelensky and Deputy Prime Minister Irina Vereshchuk have visited a hospital in Kyiv and met children evacuated from Mariupol whose parents were killed during fierce fighting in the city.

Ilya Matvienko, 10, and Kira Obedinska, 12, each lost their parents amid the ongoing Russian offensive to seize total control of the port on the Sea of Azov, Zelensky's office said.

Russian troops moved the children out of the city to Russia-controlled territory, but Ukrainian authorities managed to evacuate them to Kyiv, the presidential office said in a statement without providing further details.

Zelensky congratulated Matvienko, who had his grandmother with him at the hospital, on his birthday as he turned 10.

Mariupol's Azovstal steel plant remains the last known stronghold of Ukrainian fighters in the city, with about 1000 civilians said to be taking shelter alongside an estimated 2000 Ukrainian defenders.