At least six people were killed and power infrastructure knocked out as Russia unleashed "a massive rocket attack" across 10 regions in Ukraine on Thursday, the country’s president said Thursday.

Volodymyr Zelenskyy described the barrage that came while many people slept as an attempt by Moscow "to intimidate Ukrainians again."

"The occupiers can only terrorize 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 demoralize the civilian population.

The barrages later became less frequent, with analysts speculating Russia may have been running low on ammunition. The last massive barrage took place on Feb. 16.

U.S. intelligence doesn't believe Russia can make major territorial gains in Ukraine this year due to its large numbers of casualties, its inability to replenish its stocks of weapons and ammunition, and poor leadership and morale, Avril Haines, the director of national intelligence, told a U.S. Senate committee Wednesday. That may persuade Russian President Vladimir Putin to prolong the war, she said.

The latest missile attack left almost half of the consumers in Kyiv without heating, with temperatures at around 9 degrees Celsius (48 Fahrenheit) amid a spring thaw.

No power at nuclear plant

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 is 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 U.N. nuclear watchdog expressed alarm at the latest blackout, saying he was "astonished by the complacency" of the organization he leads, the International Atomic Energy Agency (IAEA).

"What are we doing to prevent this happening? We are the IAEA, we are meant to care about nuclear safety," he 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.

Nighttime chaos

Air raid sirens wailed through the night across Ukraine, including the capital, Kyiv, where explosions occurred in two western areas of the city. Defense systems were activated around the country.

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

Viktor Bukhta, a 57-year-old resident of a damaged residential building in Kyiv’s Sviatoshynski district, where officials said three people were wounded and apartment windows were shattered, said a missile landed nearby at about 6.45 a.m.

"We went into the yard. People were injured, they helped, first aid kits were handed out from the cars," he told The Associated Press. "Then the cars caught fire. We tried to extinguish them with car fire extinguishers. And I got a little burnt."

Kyiv’s city administration said the capital was attacked with both missiles and exploding drones. Many were intercepted but its energy infrastructure was hit.

Smoke could be seen rising from a facility in Kyiv’s Holosiivskyi district and police had cordoned off all roads leading to it.

The alarm in Kyiv was lifted just before 8 a.m., with the air raid sirens falling silent after some seven hours.

Private electricity operator DTEK reported that three of its power stations had been hit. There were no casualties, but the company said the equipment was severely damaged.

Kharkiv, Ukraine’s second-largest city, was left without running water and heating after 15 missiles hit the region, mayor Ihor Terekhov told the Ukrainian public broadcaster.

Terekhov added that electricity shortages also disrupted mobile communications and public transport in the city, as all trams and trolleybuses ground to a halt.

More death

Five people were killed in the Lviv region after a missile struck a residential area, Lviv Gov. Maksym Kozytskyi said. Three buildings were destroyed by fire, and rescue workers were combing through rubble looking for more possible victims, he said.

A sixth person was killed and two others wounded in multiple strikes in the Dnipropetrovsk region that targeted its energy infrastructure and industrial facilities, Gov. Serhii Lysak said.

In the south, Odesa Gov. Maksym Marchenko said residential buildings were hit and several power lines were damaged in strikes on his region. He said six missiles and one drone were shot down.

Ukrainian Energy Minister Herman Halushchenko condemned the missile strikes as "another barbaric massive attack on the energy infrastructure of Ukraine," saying in a Facebook post that facilities in Kyiv, Mykolaiv, Kharkiv, Zaporizhzhia, Odesa, Dnipropetrovsk and Zhytomyr regions had been targeted.

Ukrainian Railways reported power outages in certain areas, with 15 trains delayed.

Preventive emergency power cuts were applied in Kyiv, Dnipropetrovsk, Donetsk and Odesa regions, supplier DTEK said. Klitschko said 40% of consumers in Kyiv were without heating because of the emergency power cuts. Water supplies were uninterrupted, he said.