Russian troops are reportedly firing artillery at Ukrainian targets from positions within the Zaporizhzhia nuclear plant, Europe’s largest such facility, ratcheting up fears of an environmental disaster and using the plant’s six reactors as a shield against return fire.

The plant fell into Russian hands on March 4 after a dramatic assault that left the world on edge with fears of a nuclear plant disaster. But a New York Times report this week indicating that the plant has become a staging ground for rocket attacks against Ukrainian troops marked an unprecedented escalation.

Located in southeastern Ukraine near the disputed Donbas region, the Zaporizhzhia plant sits on the eastern bank of the Dnipro River in territory coveted by Moscow. According to residents of Nikopol, a Ukrainian-held stronghold across the Dnipo from the plant, rocket volleys have been coming from the inside the nuclear complex since mid-July.

“They are hiding there so they cannot be hit,” said Oleksandr Sayuk, the mayor of Nikopol, told the New York Times of the Russian troops holed up at the Zaporizhzia plant. “Why else would they be at the electrical station? To use such an object as a shield is very dangerous.”

The capture of the Ukrainian plant only days after Russia’s February 24 invasion began caused international outrage, as the battle led to a fire on the site and stoked anxiety that the cores of Zaporizhzhia’s six pressurized water reactors could be breached in the melee.

Also in danger are pools storing highly radioactive spent nuclear fuel from the plant’s reactors, which, if breached by an attack, could spread radiation throughout the local region.

The war marks the first time the world has ever witnessed the full-scale invasion of a country with an advanced nuclear power infrastructure, and the threat military hostilities pose to Ukraine’s 15 Soviet-built reactors has prompted calls for new international accords protecting reactors during wartime.

Speaking at a United Nations nuclear non-proliferation conference, US Secretary of State Antony Blinken blames Moscow for using the Zaporizhzhia plant as a “nuclear shield.”

“Of course the Ukrainians cannot fire back lest there be a terrible accident involving the nuclear plant,” Blinken told reporters after the UN talks in New York on Monday, adding that the plant seizure is “the height of irresponsibility.”

The Ukrainian army’s options for retaliating against strikes originating from inside the plant are, indeed, limited. One tactic it has used is precision strikes that avoid — as much as possible — the risk of damaging the reactors.

For instance, last month, according to the Times, Ukraine’s military intelligence agency said it struck inside the plant with a kamikaze drone that blew up an antiaircraft installation and a rocket launcher and that killed soldiers in a tent camp about 150 meters from a reactor.

But such skirmishes near the plant have renewed worries that the war will set of a radiation release in a country that is littered with delicate and dangerous nuclear sites — among them the Pridniprovskyi Chemical plant, 230 kilometers north of the Zaporizhzhia plant, and Chernobyl, which Russia occupied in March and then abandoned amid reports of radiation sickness among Moscow’s troops.

Russian shells have also struck near nuclear research facilities housing research reactors near Kharkiv, and Ukrainian nuclear officials complain that Russian missiles routinely graze the airspace over the South Ukraine nuclear power plant, located 310 kilometers east of the Zaporizhzhia plant.

It would take a direct hit from a powerful weapon to rupture the thick protective shell of a reactor core, Dmytro Orlov, a former nuclear engineer and exiled mayor of Enerhodar, the city that hosts the plant, told The Times.

But should that happen, it could cause a meltdown or explosion that would spread radiation within Ukraine and beyond, as happened during the Chernobyl disaster in 1986.

The Ukrainian operators of the plant are also a worry. Russian soldiers have subjected them to brutal interrogations, including torture and electric shocks, in efforts to ferret out partisans and informers, Orlov has told western media. About a dozen workers have vanished after being abducted, he said.

Moscow’s ultimate plans for the Zaporizhzhia plant remain uncertain. Earlier this summer, Alexey Likhachev, head of Rosatom, Russia’s state nuclear corporation, told state media that his company does not plan to take operational control of the complex — despite the reported presence of Rosatom officials at the site.

But Russian Deputy Prime Minister Marat Khusnullin on a visit to Russian-occupied Ukraine said that Kyiv may soon be forced to pay Moscow for the power produced at the plant — which amounts to a fifth of all electricity production in the country.