KYIV, Ukraine (AP) — Russia unleashed “a massive rocket attack” that hit critical infrastructure and residential buildings in 10 regions of Ukraine, the country’s president said Thursday, with officials reporting at least six deaths in the largest such nighttime attack in three weeks.

President Volodymyr Zelenskyy said the barrage that came while many people slept and knocked out power in cities across the country was an attempt by Moscow “to intimidate Ukrainians again.”

“The occupiers can only terrorise civilians. That’s all they can do,” Zelenskyy said in an online statement.

The war has largely ground to a battlefield stalemate over the winter. The Kremlin’s forces started targeting Ukraine’s power supply last October in an apparent attempt to demoralise the civilian population and compel Kyiv to negotiate peace on Moscow’s terms. The attacks later became less frequent, with analysts speculating Russia may have been running low on ammunition. The last major bombardment took place on February 16.

Overall, Russia launched 81 missiles and eight exploding Shahed drones Thursday, according to Ukraine’s chief commander of the armed forces, Valerii Zaluzhnyi. Thirty-four cruise missiles were intercepted, as were four drones, he said.

The Russian defense ministry said the attacks were in retaliation for an alleged incursion into the Bryansk region of western Russia a week earlier by what Moscow claimed were Ukrainian saboteurs. Ukraine denied the claim and warned that Moscow could use the allegations to justify stepping up its own attacks.

The Russian defense ministry said Thursday’s “massive retaliation” hit military and industrial targets in Ukraine “as well as the energy facilities that supply them.”

Private electricity operator DTEK reported that three of its power stations had been hit, causing severe damage and bringing preventive emergency power cuts in the Kyiv, Dnipropetrovsk, Donetsk and Odesa regions.

The strikes left almost half of consumers in Kyiv without heating, with temperatures at around 9 C (48 F). Kharkiv, Ukraine’s second-largest city, was left without running water, heating, trams and trolleybuses after 15 missiles hit the region, mayor Ihor Terekhov told the Ukrainian public broadcaster.

Around 150,000 households were left without power in Ukraine’s northwestern Zhytomyr region. In the southern port of Odesa, emergency blackouts occurred due to damaged power lines.

In southern Ukraine, the Zaporizhzhia Nuclear Power Plant, which is occupied by Russian forces, lost power as a result of the missile attacks, according to nuclear state operator Energoatom.

It’s the sixth time that Europe’s largest nuclear plant has been in a state of blackout since it was taken over by Russia months ago, forcing it to rely on diesel generators that can run the station for 10 days. Nuclear plants need constant power to run cooling systems and avoid a meltdown, and fears remain about the possibility of a catastrophe at Zaporizhzhia.

The head of the UN nuclear watchdog expressed alarm at the latest blackout, saying he was “astonished by the complacency” of members of the organisation he leads, the International Atomic Energy Agency.

“What are we doing to prevent this happening? We are the IAEA, we are meant to care about nuclear safety,” IAEA Director-General Rafael Mariano Grossi told its board of directors in a meeting Thursday, according to an IAEA statement.

“Each time we are rolling a dice,” he said. “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.

Power supply to the plant can be restored “within a day or two,” Leonid Oleinyk, a press secretary at Energoatom, told The Associated Press by telephone. He said emergency repairs have already begun.