The head of the United Nations nuclear watchdog has renewed appeals for a demilitarized protection zone around the Zaporizhzhia Nuclear Power Plant in Ukraine after shelling again knocked out its power, saying he was “astonished by the complacency” of the organization he leads, the International Atomic Energy Agency (IAEA).

Russian forces pounded several Ukrainian cities while people slept on Thursday, killing at least six civilians, knocking out electricity, and forcing Europe’s largest nuclear plant off the grid for a sixth time since Moscow’s invasion began last year.

The last time all power was lost at the site was on November 23, 2022, Rafael Grossi told the IAEA board of directors in a meeting on Thursday. Later in the day, Ukraine’s grid operator said the plant had been reconnected to the power grid.

“What are we doing to prevent this [from] happening? “Grossi asked at the meeting. ”We are the IAEA, we are meant to care about nuclear safety. Each time we are rolling a dice. And if we allow this to continue time after time then one day our luck will run out.”

The agency has placed teams of experts at all four of Ukraine’s nuclear power plants to reduce the risk of severe accidents.

The Zaporizhzhia plant, which has been held by Russia since March 4 of last year, can run off diesel generators for 10 days. While its six reactors have been put into various stages of shutdown for safety reasons, they still need power from the grid in order to run cooling systems and avoid a meltdown, and fears remain about the possibility of a catastrophe at Zaporizhzhia.

At the same time, Moscow, which illegally annexed the Zaporizhzhia region last October, has placed the plant under the control of its own state nuclear company, Rosatom, and has engaged in a protracted struggle with Ukrainian engineers and officials over the management of the plant. Ukrainian authorities say that some workers have been interrogated, many have been kidnapped, and at least one has been killed.

As in previous attacks, Russia and Ukraine blamed each other for the latest blackout.

Grossi has long tried to get both sides to strike a deal, pledging they would not fire at or from the plant and that heavy weapons would be removed from the site.

After the attack, the plant lost all external power supply and it relied on diesel generators — a last line of defense to prevent a meltdown from overheating reactor fuel, the IAEA confirmed.

“This morning, at around 5am local time, Zaporizhzhia Nuclear Power Plant lost all off-site power when its last remaining 750 kilovolt line was disconnected, its only remaining back up 330 kilovolt line having been damaged a few days ago and under repair,” the IAEA said in a statement on Thursday.

In his statement to the IAEA board, Grossi stressed: “This is the sixth time – let me say it again sixth time – that ZNPP has lost all off-site power and has had to operate in this emergency mode. Let me remind you – this is the largest nuclear power station in Europe. What are we doing? How can we sit here in this room this morning and allow this to happen? This cannot go on.”

The first big volley of missile attacks since mid-February shattered the longest period of comparative calm since Russia began a campaign to attack Ukraine’s civil infrastructure five months ago.

President Volodymyr Zelenskyy said infrastructure and residential buildings in 10 Ukrainian regions had been hit.