Adviser to the mayor of Mariupol, Petr Andryushchenko, reportedly said the Russian occupation authorities were already beginning to quarantine parts of the city.
He warned tens of thousands of deaths from a cholera epidemic was "unfortunately a potentially real scenario for occupied Mariupol".
He said: “We are watching the city shut down. In addition, information has begun to come in that the Russian side in Rostov-on-Don has prepared separate infectious diseases departments, where they are preparing to receive their own military who may end up [in the hospital] as a result of the epidemic.
"And the word cholera is heard inside the city, among the occupying authorities and their representatives."
There have been concerns about disease outbreaks in the occupied city due to due to mass graves following heavy casualties in the region and lack of access to clean drinking water.
Photographs published by Mariupol council have shown people queuing for water at open wells contaminated with rubbish.
Cholera is a bacterial disease usually spread through contaminated water, which causes severe diarrhoea and dehydration.
Last month Dorit Nitzan, regional director for Emergency Situations of the World Health Organisation, warned at a press conference in Kyiv about a possible outbreak of cholera in Mariupol.
Most utilities in Mariupol are still not functioning due to Russian shelling, with gas, electricity and water supplies all down.
Around 200,000 residents are believed to have escaped the city, with around 22,000 killed in the fighting. Around 100,000 remain, but many of those are believed to be older, vulnerable people.
It comes as Russia claimed it had occupied large swathes of eastern Ukraine after a relentless, weeks-long barrage and the recent deployment of more troops.
Russian defence minister Sergei Shoigu said Moscow’s forces had “liberated” 97 per cent of the Luhansk region.
Russia appears determined to capture the entire eastern Donbas part of Ukraine, which is made up of the Donetsk and Luhansk regions. That goal appears to be its most immediate ambition in Ukraine.
But while the Kremlin’s forces have superior firepower, the Ukrainian defenders — among them the country’s most well-trained forces — are entrenched and have shown the capability to counterattack.
Mr Shoigu claimed Russian forces had seized the residential quarters of Sievierodonetsk and were fighting to take control of an industrial zone on its outskirts and the nearby towns.
Sievierodonetsk, the administrative centre of the Luhansk region, has recently been the focus of the Russian offensive.
Sievierodonetsk and nearby Lysychansk are the only two Donbas cities holding out against the Russian invasion, which is being helped by local pro-Kremlin forces.
Mr Shoigu said the Russian troops were pressing their offensive toward the town of Popasna and noted they had taken control of Lyman and Sviatohirsk and 15 other towns in the region.