KYIV, Ukraine (AP) — A barrage of more than 80 Russian missiles and a smaller number of exploding drones hit residential buildings and critical infrastructure across Ukraine on Thursday, killing six people and leaving hundreds of thousands without heat or electricity.

The largest such attack in three weeks also put Europe’s largest nuclear plant at risk by knocking it off the power grid for nearly half of the day before it was reconnected. Because nuclear reactors need constant power to run cooling systems to avoid a meltdown, the latest power loss at the Zaporizhzhia plant again raised the spectre of a nuclear catastrophe.

Air raid sirens wailed through the night as the attacks targeted a wide swath of the country, including western Ukraine, which is far from the front lines. President Volodymyr Zelenskyy said the assault that came while many people slept was “another attempt by the terrorist state to wage war against civilization.”

The Russian Defense Ministry said the strikes were in retaliation for a recent incursion into the Bryansk region of western Russia 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 assaults.

READ: Russian missile barrage slams into cities across Ukraine

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, and analysts speculated that Russia may have been running low on ammunition. The last major bombardment was February 16.

The head of the UN nuclear watchdog agency said the Russia-controlled Zaporizhzhia nuclear plant lost all external power for 11 hours after its last remaining power line was disconnected following reports of the missile strikes. Rafael Grossi of the International Atomic Energy Agency emphasised that the incident “again demonstrated how fragile and dangerous the situation is” for the plant.

Overall, Russia launched 81 missiles and eight exploding Iranian-made Shahed drones Thursday, according to Ukraine’s chief commander of the armed forces, Valerii Zaluzhnyi. Thirty-four missiles were intercepted, as were four drones, he said. The mixture of munitions makes it harder for air defences to cope with the onslaught, military analysts say.

Among the weapons were six hypersonic Kinzhal cruise missiles, which are among the most sophisticated weapons in the Russian arsenal, Ukrainian air force spokesman Yurii Ihnat said. Ukraine says its air defences cannot intercept them.

The Russian Defense Ministry said the barrage hit military and industrial targets in Ukraine “as well as the energy facilities that supply them.”

The missile strikes took no toll on the army’s combat capability, but they played “on the nerves of the civilian population of Ukraine,” Ukrainian military analyst Oleh Zhdanov told The Associated Press.