Cholera and other deadly diseases could kill thousands of people in the southern Ukrainian city of Mariupol as corpses lie uncollected and summer brings warmer weather, its mayor said on Friday.

Mayor Vadym Boichenko said wells had been contaminated by the corpses of people killed during weeks of Russian bombardment and siege, and that the collection of bodies by the city's Russian occupiers was proceeding slowly.

"There is an outbreak of dysentery and cholera. This is unfortunately the assessment of our doctors: that the conflict which took over 20,000 residents, unfortunately, with these infection outbreaks, will claim thousands more Mariupolites," he told national television.

Boichenko, who is based outside Mariupol, said the city had been placed into quarantine.

Ukraine says about 100,000 people are now in Mariupol, a once vibrant city that had a population of about 430,000 before the conflict but is now an urban wasteland.

The World Health Organization warned last month of a possible cholera outbreak in Mariupol. 

Britain's Defence Ministry said on Friday there was a risk of a major cholera outbreak in Mariupol because medical services were probably near collapse.

Russia is struggling to provide basic public services to the population in Russian-occupied territories, it said.

Russia did not immediately comment on the assertions by Boichenko or the British defence ministry. Moscow says its "special military operation" is intended to disarm and "denazify" Ukraine. Kyiv and its allies call it an unprovoked conflict to capture territory.

(With input from Reuters)