Thousands of civilians may have died in the besieged port city of Mariupol in southern Ukraine since bombing began four weeks ago, the head of the UN human rights mission told Reuters on Tuesday, providing its first estimate.

Nearly 5,000 people, including about 210 children, have been killed in Mariupol since Russian forces laid siege to it a month ago, a spokesperson for Mayor Vadym Boichenko said on Monday.

His office said 90 percent of Mariupol's buildings had been damaged and 40 percent...