Russian forces have seized the greatest part of a strategic city in eastern Ukraine, a move that would mark an important step towards overrunning the heavily-contested Donbas region.

The forces were now in control of almost 80 percent of Severodonetsk, the governor of the Luhansk region, Sergiy Gaiday, told The Associated Press via telephone on Tuesday.

Together with Donetsk, Luhansk forms the Donbas, which has been the scene of a years-long conflict between pro-Russian separatists and Ukrainian forces.

In 2014, the two regions declared themselves new republics, refusing to recognize Ukraine’s Western-backed government.

In February, Russian President Vladimir Putin ordered a military operation in Ukraine aimed at “demilitarizing” the Donbas.

Putin said the mission was aimed at “defending people who for eight years were suffering persecution and genocide by the Kiev regime.”

Gaiday said Ukrainian forces had now been pushed to the industrial outskirts of Severodonetsk, adding that about 500 civilians were still sheltering in the Azot chemical plant.

“There is still an opportunity for the evacuation of the wounded, [as well as] communication with the Ukrainian military and local residents,” he said, noting that Russian soldiers had not yet completely blocked off the city.

Meanwhile, Russia said Tuesday it will open a humanitarian corridor to allow civilians encircled in the chemical plant to leave the area.

The Russian Defense Ministry said the civilians will be evacuated to the Lugansk region, which is being controlled by the pro-Russia forces.

After the capture of Severodonetsk, Russian forces are expected to take control of Slovyansk and Kramatorsk to capture the entire Donbas.

Ukraine pleads for Western arms

Ukrainian President Volodymyr Zelensky has urged Western countries to provide heavy weapons as quickly as possible.

On Tuesday, Ukraine’s Deputy Defense Minister Anna Malyar said that the country only received 10 percent of the arms it had requested from its Western allies.

Ukrainian forces were suffering “painful” losses in the fight against Russian troops in the east as they awaited the delivery of the promised weapons, Zelenskiy said.

Ukraine needs modern anti-missile weapons now, he said, adding there could be no justification for partner countries to delay delivery.

“We keep telling our partners that Ukraine needs modern anti-missile weapons,” Zelenskiy said. “Our country does not have them yet at a sufficient level, but it is in Ukraine and right now that there is the greatest need for such weapons. Procrastination in providing them cannot be justified.”