A Russian-backed separatist official said that at least 1,200 civilians, including 127 children, are being “held hostage” at a chemical plant in the besieged Ukrainian city of Severodonetsk.

Speaking to the Russian state news agency TASS, the pro-Russian Luhansk People’s Republic’s interior minister, Vitaly Kiselev, alleged that Ukrainian forces were keeping civilians at the Azot plant “against their will.”

“They have been held there against their will for quite a while,” he said on Wednesday, according to an English translation of the news release. “There is no water, no food, no medicines.” According to the official, it is believed that the civilians are in the building’s bunkers and underground tunnels.

The city of Severodonetsk remains at the center of Russia’s current major assault in what has been coined the “battle for the Donbas” — a breakaway region that is supported by Russia.

The bombardment of the Azot plant echoes the siege at a similar chemical plant in Mariupol last month. Thousands of Ukrainian fighters and civilians took shelter in the plant until mid-May, when soldiers finally surrendered.

The region of Luhansk still remains held by the Ukrainian government, but Severodonetsk, a major city in the Luhansk region, now has 80% of its area in the hands of the Russian army.

However, Serhiy Haidai, the Ukrainian governor of Luhansk, said that about 500 civilians were sheltering in the Azot plant, along with 40 children.

In a statement released on June 6 on behalf of the Ukrainian businessman who owns the chemical plant, American lawyer Lanny Davis said that around 800 people were sheltered beneath the plant. That included about 200 of the 3,000 employees who had worked there. The workers had stayed behind to protect “as best as possible what is left of the plant's highly explosive chemicals.”

Meanwhile, Russia said it would set up a humanitarian corridor for the people trapped inside the plant. The operation was to run from 8 a.m. to 8 p.m. Moscow time on Wednesday.

The ultimatum was announced by the head of Russia’s National Defense Management Center. Mikhail Mizintsev told Ukrainian soldiers to “stop their senseless resistance and lay down arms.”

But according to reports, Ukrainian forces showed no sign of slowing down their defense in the eastern industrial city. “They’re defending Severodonetsk and not letting them advance to Lysychansk,” Haidai said, referring to a nearby city still under Ukrainian control. “Nevertheless, the Russians are close and the population is suffering, and homes are being destroyed.”

On Monday, Eduard Basurin, the deputy leader of the Donetsk People's Republic, or the People’s Militia, told reporters that the remaining Ukrainian soldiers were trapped in Severodonetsk after the last bridge entering the city was destroyed.

“Severodonetsk is actually blocked after they blew up the last bridge that connected it with Lysychansk yesterday,” Basurin said, according to the Associated Press. “Therefore, the Ukrainian military units that are stationed there remain there forever.” He added, “They have two options: Either follow the example of their colleagues and surrender, or die. They have no other option.”