Photo used for illustrative purpose.

Mariupol is surrounded by Russian forces, who have bombarded the port city despite promises of a ceasefire to allow civilians to be evacuated.

It is not known how long the girl, named only as Tanya, had been under the ruins of her home before she died, but her mother was found dead at the scene.

"In the last minutes of her life she was alone, exhausted, frightened and terribly thirsty. This is just one of the many stories of Mariupol, which has been surviving a blockade for eight days," Mayor Vadym Boychenko said on the city's Telegram channel.

Residents of Mariupol have been cut off from electricity, water and gas. Communications are disrupted and attempts to deliver food and medicine have failed.

President Volodymyr Zelensky noted the death of the girl in a video message released to urge Ukraine's Western partners to do more to help Kyiv counter Moscow's invasion.

"Mariupol was surrounded, blocked, is being exhausted, tortured," he declared.

"For the first time in dozens of years, perhaps for the first time since the Nazi invasion, a child died of dehydration. Hear me, today, dear partners! A child died of dehydration in 2022!"

Agence France-Presse