Ukrainian authorities reported explosions in at least three cities Friday, saying Russia has launched a major missile attack on energy facilities and infrastructure.

Local authorities on social media reported explosions in the capital, Kyiv, southern Kryvyi Rih and northeastern Kharkiv as air raid alarms sounded across the country, warning of a new barrage of the Russian strikes that have occurred intermittently since mid-October, The Associated Press said.

Kharkiv Mayor Ihor Terekhov said on the Telegram social media app that the city is without electricity. Kharkiv regional governor Oleh Syniehubov reported three strikes on the city’s critical infrastructure.

Kyrylo Tymoshenko, a top official in President Volodymyr Zelenskyy’s office, reported a strike on a residential building in Kryvyi Rih, warning on Telegram: “There may be people under the rubble.” Emergency services were on site, he said.

Kyiv Mayor Vitali Klitschko reported explosions in at least four districts, urging residents to go to shelters.

“The attack on the capital continues,” he wrote on Telegram. Subway services in the capital were suspended, he said, as city residents flocked inside its tunnels deep underground to seek shelter.

Ukrzaliznytsia, the national railway operator, said power was out in a number of stations in the eastern and central Kharkiv, Kirovohrad, Donetsk and Dnipropetrovsk regions, due to damage to the energy infrastructure. But trains continued to run by switching from electric power to steam-engine power, which had been readied as a backup.

Such strikes targeting energy infrastructure have been part of a new Russian strategy to try to freeze Ukrainians into submission after key battlefield losses by Russian forces in recent months. But some analysts and Ukrainian leaders say such an onslaught has only strengthened the resolve of Ukrainians to face up to Russia’s invasion that began on Feb. 24.

The previous such round of massive Russian air strikes across the country took place on Dec. 5. Ukrainian authorities have reported some successes in intercepting and downing incoming missiles, rockets and armed drones.