By Kosta Gak, Helen Regan, Svitlana Vlasova, Victoria Butenko, Sana Noor Haq, CNN

Kyiv, Ukraine (CNN) — Moscow unleashed a lethal barrage on Ukraine early Tuesday, hitting the capital Kyiv and the central city of Dnipro in a broad-ranging offensive that inflicted one of the deadliest attacks for months.

At least 23 people were killed in the overnight assault, including seven people in Kyiv and 16 others in Dnipro, according to Ukrainian authorities. Two of those were children and one was a deputy fire chief – Maj. Anton Yarmolenko – who was on call at the time. In total, more than 100 people were wounded.

In the capital alone, more than 41,000 residents sought refuge in underground stations, Kyiv Metro authorities said. It marked the highest number of people to have taken shelter in the metro during a night-time air raid alert in recent years, mirroring the scale of the bombing campaign.

More than 600 drones and dozens of missiles, such as advanced hypersonics, were fired on Ukraine, according to the military, hitting key civilian infrastructure. Within Kyiv, five medical facilities and several residential and commercial blocks were damaged or destroyed, sparking fires, burning cars and knocking debris onto a kindergarten, authorities reported.

United Nations Secretary-General António Guterres “strongly condemns” the attacks, his spokesperson Stephane Dujarric said Tuesday. “Once again, we emphasize that attacks against civilians and civilian infrastructures are prohibited under international humanitarian law and must end immediately,” he said.

Moscow launched the onslaught against threats of heightened aggression, citing a wave of Ukrainian near-daily attacks on Russian oil assets that tanked fuel stocks and exacerbated economic woes. Last month the Kremlin warned of “consistent, systematic strikes” on “specific sites” in Ukraine – prompting Kyiv to push back against “Russian blackmail and threats.”

Ukrainian President Volodymyr Zelensky warned Tuesday of the possibility of further attacks. “According to ⁠our intelligence, another large-scale attack may occur tonight,” ‌he ⁠said in his nightly video address. “Please, I strongly ⁠urge you to pay ⁠attention ‌to air raid alerts.”

He told citizens to exercise greater caution as due to a limited supply of air defenses, Ukrainian forces are not able “to shoot down a significant proportion of the missiles” being fired by Russia.

Between January and May, Ukrainian troops struck 15 Russian oil refineries, knocking out 40% of Russia’s main oil refining capacity, Ukrainian President Volodymyr Zelensky said Monday. CNN cannot independently verify the report.

The rise in aerial attacks by both sides comes amid a relative stalemate on the ground. After more than four years, Russia’s full-scale invasion has morphed into a grinding war of attrition where soldiers are being killed en masse, financial losses are piling up and Kyiv has started to liberate more land than Moscow has seized. Last month, Ukrainian troops largely stalled a renewed Russian offensive, according to the Institute for the Study of War (ISW), a US-based conflict monitor.

Putin ‘trying to erase Ukraine into a graveyard’

Charred cars were buried under rubble and people trudged through a debris-laden landscape in Kyiv on Tuesday. In one video clip, an elderly couple could be seen forking out the remnants of their flat, where the window facade was completely shattered.

Several Ukrainians described reams of dark smoke curling through the neighborhood early Tuesday, as they tried to recover their belongings while thunderous bombardment hammered overhead.

One man was fumbling in the dark, after he awoke to heavy explosions that blew open the entrance of his house. “It was so powerful that everything came flying at us,” Mykhailo Sartynski told CNN. “I was terrified for my wife… Everything was on fire.”

Another resident, Karina Kasamara, recalled an initial blast “shattering the windows, balconies, and everything.” “The dog ran out into the hallway and then all hell broke loose,” she said “(Russian President Vladimir Putin) is trying to erase Ukraine into a graveyard. It seems to me that’s where it’s all heading.”

Kyiv’s air defenses appeared to be less active during a ballistic missile strike around 7 a.m. local time, with CNN producers in the city center hearing ongoing explosions, but not the firing of counter-air systems.

The mayor of the capital, Vitali Klitschko, described the overnight assault as a “massive enemy attack.” Emergency crews fear people are trapped under the rubble of a multi-story apartment block in Podilsky district, following a “double tap” Russian strike, according to Klitschko.

Nearby, in Bucha, three homes were damaged, as well as warehouse facilities and non-residential buildings, according to the Kyiv region’s governor, Mykola Kalashnyk.

Further east, Russian attacks were reported in Kharkiv, where 14 people were wounded, including a child, Ukraine’s State Emergency Service said.

Altogether, Russia fired 656 drones and 73 missiles at Ukraine overnight, according to Ukrainian Air Force figures, which said the vast bulk of the drones and just over half of the missiles were shot down.

Russia fired eight of its advanced hypersonic Zircon missiles toward Ukraine, the air force said, but none were intercepted. Experts have previously told CNN the Zircon missiles are near impossible to shoot down.

The main targets of the strike were Kyiv, Dnipro, Kharkiv, Zaporizhzhia, and Poltava, according to the air force statement.

In a statement on Telegram, Russia’s Ministry of Defense said its “massive strike” targeted Ukrainian defense, military, fuel and transport facilities in several key regions with“long-range precision-guided air, land, and sea-based weapons, including hypersonic aeroballistic missiles and unmanned aerial vehicles.”

Last month, Moscow leveled blame on Kyiv for an attack on a college dormitory in Starobilsk, a Russian-occupied town in eastern Luhansk. At the time, Ukraine insisted its forces solely target “military infrastructure.” But late Monday, Putin warned the Starobilsk strike had added “a new dimension to the conflict.” The Kremlin echoed that sentiment on Tuesday.