Russia-Ukraine updates: 2 US veterans who joined Ukrainian forces missing

The fight for the eastern Ukrainian city of Severodonetsk will determine the fate of the wider Donbas region, according to Ukrainian President Volodymyr Zelenskyy.

"Severodonetsk remains the epicenter of the confrontation in Donbas. We defend our positions, inflict significant losses on the enemy," Zelenskyy said late Wednesday in his nightly address. "This is a very fierce battle, very difficult. Probably one of the most difficult throughout this war. I am grateful to everyone who defends this direction. In many ways, the fate of our Donbas is being decided there."

After launching an invasion of neighboring Ukraine on Feb. 24, Russian forces failed to take control of the capital, Kyiv, and other major government centers as they faced tough resistance from Ukrainian troops. Russian forces then switched attention to Donbas, which comprises the self-proclaimed republics controlled by Moscow-backed separatists in eastern Ukraine's Luhansk and Donetsk oblasts.

Severodonetsk, an industrial hub, is the largest city still held by Ukrainian troops in contested Donbas. In recent days, Russian forces have encircled the city as they advanced in the region, creating a pocket that could trap Ukrainian defenders there and in the neighboring city of Lysychansk.

Severodonetsk and Lysychansk are the last major cities in the Luhansk area still controlled by Ukraine. Last week, the U.K. Ministry of Defense said in an intelligence update that Russian forces had seized most of Severodonetsk, but that the main road into the pocket likely remained under Ukrainian control.

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."