The port city of Mariupol in eastern Ukraine is facing the risk of a cholera epidemic amid the destruction of water supplies and sanitation during the Russian invasion, city officials and health agencies warn.

"The risk of cholera is very high, like red, red level," Petro Andriushchenko, an advisor to Mariupol’s mayor, told ABC News, adding that the municipality could not provide an estimation of the number of infected cases due to lack of proper access to the occupants amid the occupation by Russian forces.

While the warnings have intensified in the past few days, Mariupol Mayor Vadym Boychenko said on Telegram last month that due to problems with water supply, the Russian occupied city is threatened by an infectious catastrophe and more than 10,000 people may die by the end of the year.

The deteriorating water, sanitation and hygiene infrastructure has set an alarmingly high risk of an outbreak, according to a report in April from the World Health Organization's Health Cluster Ukraine agency.

The warming spring and summer weather will likely increase transmission, the report said.

"The weather is hot. There are still dead bodies on the streets of the city -- especially under the debris of residential buildings. In some blocks, it is impossible to walk by -- due to the stench of rotten human flesh. There was no rain for a while, and it is getting hotter," a resident of Mariupol, who did not want to be named for security concerns, told ABC News.

-ABC News' Somayeh Malekian and Yuriy Zaliznyak

The Russian-occupied city of Mariupol, Ukraine, is facing a "catastrophic lack of medical staff," Petro Andryushchenko, an adviser to the mayor of Mariupol, said on the Telegram app.

He said Russians are trying to convince locals who are over 80 years old to go back to work at hospitals.

He warned, "In this state of medicine, any infectious disease turns into a deadly epidemic."

A meeting between Russian President Vladimir Putin and Ukrainian President Volodymyr Zelenskyy is not currently possible, the Kremlin said.

When asked about a recent comment from Zelenskyy that he's willing to meet with Putin, Kremlin spokesman Dmitry Peskov said, "Our position is well-known here: good preparations need to be made for a top-level meeting. We know that the Ukrainian side has withdrawn from the negotiation track, and therefore it is currently not possible to prepare for this sort of top-level meeting."

Ukrainian troops defending the eastern city of Sieverodonetsk are "holding," despite attacks in three directions from Russian forces, the U.K. Ministry of Defense said Wednesday in an intelligence update.

"Russia continues to attempt assaults against the Sieverodonetsk pocket from three directions although Ukrainian defences are holding," the ministry said. "It is unlikely that either side has gained significant ground in the last 24 hours."

Sieverodonetsk, an industrial hub, is the largest city still held by Ukrainian troops in the contested Donbas region of Ukraine's east, which comprises the self-proclaimed republics in Luhansk and Donetsk oblasts. In recent days, Russian forces have encircled the city as they advanced in Donbas, creating a pocket that could trap Ukrainian defenders there and in the neighboring city of Lysychansk.

Sievierodonetsk and Lysychansk are the last major cities in the Luhansk area still controlled by Ukraine.

Last week, the U.K. Ministry of Defense said Russian forces had seized most of Sieverodonetsk, but that the main road into the pocket likely remained under Ukrainian control.

With the frontage of the Russian-occupied territories of Ukraine stretching for over 300 miles, "both Russia and Ukraine face similar challenges in maintaining a defensive line while freeing up capable combat units for offensive operations," according to the ministry.

"While Russia is concentrating its offensive on the central Donbas sector, it has remained on the defensive on its flanks," the ministry said in its intelligence update Wednesday. "Ukrainian forces have recently achieved some success by counter-attacking in the south-western Kherson region, including regaining a foothold on the eastern bank of the Ingulets River."

Russia's invasion of Ukraine has already engaged around 330,000 Russian personnel, Brigadier General Oleksiy Hromo of the General Staff of the Ukrainian Armed Forces told local media on Thursday.

“[Russian] groups at various axes are close to 150,000 strong. If we add their air and sea components, there are about 220,000 military personnel,” Hromo said. Russia also deployed “units of the national army combat reserve, the federal service of guard forces, and mobilization units,” the Brigadier General added.

Hromo's sobering account came on the back of claims made by a top Ukrainian official earlier on Wednesday that revealed mounting Ukrainian casualties. Up to 1,000 Ukrainian soldiers are being killed or wounded each day in the Donbas region of eastern Ukraine, with 200 to 500 killed on average and many more wounded, said David Arakhamia, who heads the presidential faction in the Ukrainian parliament.

In early June, Ukrainian President Volodymyr Zelenskyy said daily fatalities among Ukrainian ranks at the Donbas front were between 60 to 100 troops.

Arakhamia, one of Zelenskyy's closest advisers who oversees Ukraine's stalled negotiations with Russia, has been leading a Ukrainian delegation in Washington this week in a bid to lobby the Biden administration and Congress.

Arakhamia's team want to see the pace of weapons shipments increased and is pushing for a designation of Russia as a state sponsor of terrorism.

-ABC News' Edward Szekeres, Yuriy Zaliznyak, Yulia Drozd and Max Uzol