Russian troops are holding 500 people hostage in a hospital in the port city of Mariupol, according to Donetsk governor Pavlo Kirilenko.
It is believed Russian soldiers pushed around 400 people from neighbouring buildings to the hospital.
Mr Kirilenko wrote in Telegram that around 100 doctors and patients are also understood to be in the hospital.
According to Kirilenko, the troops forced them inside the hospital as 'human shields', not letting anyone go outside.
Mr Kirilenko said it is impossible to leave the hospital as they "shoot a lot."
The hospital has previously been the target of an attack sustaining substantial damage to its main building.
Health workers have tried their best to make do with what they have as they still have to treat patients.
They have constructed makeshift beds in the basement, according to Kirilenko.
He asks the world to respond to the "gross transgressions of the rules and customs of war, these horrific crimes against humanity."
The Ukrainian defence said Russia was trying to isolate the city from the west and east's outskirts.
The General Staff wrote on Facebook that it was a "heavy loss."
Ukraine's deputy prime minister Olha Stefanishyna said in an update that Russian forces were now shooting from the captured hospital, threatening the humanitarian corridor.
So far, about 20,000 civilians have managed to leave the besieged city in private cars, as Mariupol residents have been trapped in the southern port city by Russian shelling without heating, electricity and running water.
Official Ukrainian estimates last week suggested at least 200,000 people were in urgent need of evacuation.
Ms Stefanishyna said efforts are being made to open a humanitarian corridor again today.
