KYIV, UKRAINE -- Rescuers searched the rubble of Ukraine's biggest children's hospital on Tuesday for more dead and wounded, a day after authorities say a Russian missile leveled a wing of the Kyiv facility during a massive daytime barrage that killed at least 42 people throughout the country.

Ukrainian President Volodymyr Zelenskyy said on the social platform X that 64 people were hospitalized in the capital, in addition to 28 in Kryvyi Rih and six in Dnipro -- both cities in central Ukraine.

It was Russia's heaviest bombardment of Kyiv in almost four months and one of the deadliest of the war, hitting seven of the capital's 10 districts, according to Ukrainian officials. The strike on the Okhmatdyt children's hospital, which interrupted open-heart surgery and forced young cancer patients to take their treatments outdoors, drew international condemnation.

The 10-story hospital, Ukraine's largest medical facility for children, was caring for 627 patients at the time of the attack, according to the country's health minister.

"The building where we conducted dialysis for children with kidney failure or acute intoxication is ruined entirely," the hospital's director general, Volodymyr Zhovnir, told reporters, estimating the overall damage at $2.5 million.

Danielle Bell, the head of a United Nations team tracking human rights violations in Ukraine, said at least two people were killed at the hospital and about 50 were injured, including seven children. There would have been a lot more casualties if patients hadn't been taken to a bunker when air raid sirens first sounded, she added.

Zhovnir said one of the two people killed at the hospital was a female doctor who had taken children to the shelter before going back to check whether anyone had been left behind.

Pediatric surgeon Oleh Holubchenko told The Associated Press he was operating on a baby with congenital face defects and that he and his team decided to continue despite the blast of air sirens. "We couldn't stop halfway through," he said.

When the missile struck, the shock wave sent him hurtling across the operating theater. Shrapnel caused him minor injuries and pierced the infant's ventilator. The baby, still with an open wound, had to be transported to another Kyiv hospital, where they finished the surgery.

Authorities were working to restore the hospital's power and water supply as Kyiv city administrators declared Tuesday an official day of mourning. Entertainment events were prohibited and flags were lowered in the capital.

UN Security Council members confronted Russia over the hospital strike, condemning Moscow during an emergency meeting Tuesday chaired by the Russian ambassador, Vassily Nebenzia.

Russia denied responsibility, insisting it doesn't attack civilian targets in Ukraine despite abundant evidence to the contrary, including AP reporting. Kremlin spokesman Dmitry Peskov pointed to a Russian Defense Ministry statement that blamed the hospital strike on a Ukrainian air defence missile.

Nebenzia repeated that position during the Security Council meeting, saying: "If this had been a Russian strike, there would have been nothing left of the building,"

Bell, the head of the UN team, dismissed the Russian argument. She said an assessment of video footage and findings on site indicated the hospital "took a direct hit, rather than receiving damage due to an intercepted weapons system."

The hospital likely was struck by a Russian Kh-101 cruise missile, Bell said. Ukrainian officials said the same.

Investigators with the International Criminal Court in The Hague visited the hospital Tuesday to collect evidence for possible war crimes charges, the court said. Hospitals, along with other civilian sites, are protected under international humanitarian law.

Pope Francis on Tuesday expressed his 鈥済reat sorrow鈥 about the hospital attack and his 鈥渄eep shock鈥 at the escalation of violence.

Over the past year, Moscow has been honing tactics to break through Ukraine鈥檚 air defences, said Alexander Kovalenko, a military analyst from Information Resistance, a Kyiv-based think tank.

Russia used a large number of various types of missiles Monday that, arriving together, made it hard for Ukrainian air defences to intercept them, he said.

鈥淭hey have been experimenting with different kinds of rockets since 2023 to find the perfect combination to break through our air defence algorithms,鈥 Kovalenko told the AP.

Russian forces have also programmed missiles to maneuver mid-air, according to Kovalenko, who added that during Monday鈥檚 attack, a missile first appeared to be heading farther west before it swerved back to strike Kyiv.

The bodies of three more people were found Tuesday under the rubble of a residential building in the Shevchenkivskyi district of Kyiv, officials said, bringing the death toll in the single building to 10.

The Russian onslaught Monday came on the eve of a NATO summit in Washington where alliance countries are expected to pledge new military and economic support for Ukraine.

Russian President Vladimir Putin, meanwhile, was hosting India鈥檚 Prime Minister Narendra Modi in Moscow. New Delhi鈥檚 importance as a key trading partner has grown since the Kremlin sent troops into Ukraine in February 2022.

Zelenskyy was deeply critical of Modi鈥檚 visit, saying on X late Monday: 鈥淚t is a huge disappointment and a devastating blow to peace efforts to see the leader of the world鈥檚 largest democracy hug the world鈥檚 most bloody criminal in Moscow on such a day.鈥

Meanwhile, Russian military and regional officials said Tuesday that Ukrainian drones targeted six Russian regions overnight, in what appeared to be a bigger-than-usual aerial attack by Kyiv's forces.

Russia鈥檚 Defense Ministry said air defence systems in five Russian southern and western regions 鈥渄estroyed and intercepted鈥 a total of 38 Ukrainian drones.

Jamey Keaten in Geneva contributed.