KYIV, Ukraine (AP) — Russian missiles blasted cities across Ukraine on Monday, damaging the country’s largest children’s hospital and other buildings in a fierce assault that interrupted heart surgeries and forced young cancer patients to take their treatments outdoors. At least 31 people were killed, officials said.

The daytime barrage targeted five Ukrainian cities with more than 40 missiles of different types, President Volodymyr Zelenskyy said on social media. Ukraine’s air force said it intercepted 30 missiles. More than 150 people were wounded.

It was Russia’s heaviest bombardment of Kyiv in almost four months, hitting seven of the city’s 10 districts. At least seven people were killed in the capital, including two staff members at the hospital. Strikes in Kryvyi Rih, Zelenskyy’s birthplace in central Ukraine, killed 10.

The attack on the Okhmatdyt children’s hospital caused debris to fall into heart patients’ open chests in the middle of surgery. Cancer patients had their beds wheeled into parks and onto the streets.

“It is very important that the world should not be silent about it now and that everyone should see what Russia is and what it is doing,” Zelenskyy said.

Russia denied attacking the hospital and said the strikes hit military targets.

The assault unfolded a day before Western leaders who have backed Ukraine were scheduled to begin a three-day NATO summit in Washington to consider how they can reassure Kyiv of the alliance’s unwavering support and offer Ukrainians hope that their country can survive Europe’s biggest conflict since World War II.

Zelenskyy said during a visit to Poland that he hopes the summit will provide more air defense systems for Ukraine.

In a statement, U.S. President Joe Biden called Monday’s missile strikes “a horrific reminder of Russia’s brutality.”

“It is critical that the world continues to stand with Ukraine at this important moment and that we not ignore Russian aggression,” the statement said.

At the hospital in Kyiv, rescuers searched for victims under the rubble of a partially collapsed, two-story wing of the facility. At the main 10-story building, windows and doors were blown out, and walls were blackened. Blood was spattered on the floor in one room. The intensive care unit, operating theaters and oncology departments all were damaged, officials said.

At the time of the strike, three heart operations were being performed, leading to the contamination of the patients’ open chests with blast debris, Health Minister Viktor Liashko said.

The hospital lost water, light and oxygen, and the patients were transferred to other hospitals, he told Ukrainian television.

Rescuers formed a line, passing bricks and other debris to each other as they sifted through rubble. Smoke rose from the building, and volunteers and emergency crews worked in protective masks.

Some mothers carried their children away on their backs, while others waited in the courtyard with their children as calls to doctors’ phones rang unanswered.

A few hours after the initial strike, another air-raid siren sent many of them hurrying to the hospital’s shelter. Led by a flashlight through the shelter’s dark corridors, mothers carried their bandaged children in their arms, and medical workers carried other patients on gurneys. Volunteers handed out candy to try to calm the children.

Marina Ploskonos said her 4-year-old son had spinal surgery Friday.

“My child is terrified,” she said. “This shouldn’t be happening, it’s a children’s hospital,” she said, bursting into tears.

“Among the victims were Ukraine’s sickest children,” said Volker Türk, the U.N. human rights commissioner. A U.N. team visited the hospital shortly after it was hit and saw the children receiving cancer treatments in hospital beds set up outdoors, he added.

“This is abominable, and I implore those with influence to do everything in their power to ensure these attacks stop immediately,” Türk said.

Kyiv city administrators declared July 9 a day of mourning, when entertainment events are prohibited and flags are lowered.

Ukraine’s Security Service said it found wreckage from a Russian Kh-101 cruise missile at the site and opened proceedings on war crime charges. The Kh-101 is an air-launched missile that flies low to avoid detection by radar. Ukraine said it shot down 11 of 13 Kh-101 missiles launched Monday.

U.N. Secretary-General Antonio Guterres called Monday’s missile strikes “particularly shocking,” U.N. spokesman Stephane Dujarric said.

The U.N. Security Council scheduled an emergency meeting on the attacks for Tuesday at the request of France and Ecuador. Russia, which holds the council’s rotating presidency this month, will preside at the meeting.

The International Criminal Court’s founding charter says it is a war crime to intentionally attack “hospitals and places where the sick and wounded are collected, provided they are not military objectives.”

Late last month, the court issued arrest warrants for Russia’s former defense minister and its military chief of staff for attacking Ukraine’s electricity network.

Russia’s Defense Ministry said the strikes targeted Ukrainian defense plants and military air bases and were successful. It denied aiming at any civilian facilities and claimed without evidence that pictures from Kyiv indicated the damage was caused by a Ukrainian air defense missile.

Since early in the war that is well into its third year, Russian officials have regularly claimed that Moscow’s forces never attack civilian infrastructure in Ukraine, despite abundant evidence to the contrary, including Associated Press reporting.

More than 1,600 medical facilities have been damaged since the start of the war and 214 have been ruined completely, according to Ukrainian Health Ministry statistics published last month.

Col. Yurii Ignat of the Ukrainian air force said Russia has been improving the effectiveness of its airstrikes, equipping its missiles with enhancements, including so-called heat traps that evade air-defense systems.

In Monday’s attack, the cruise missiles flew as low as 50 meters (160 feet) off the ground, making them harder to hit, he said in comments sent to AP.

About three hours after the first strikes, more missiles hit Kyiv and partially destroyed a private medical center. Four people were killed there, Ukraine’s Emergency Service said.

In the capital’s Shevchenkivskyi district, a three-story section of a residential building was destroyed. Emergency crews searched for casualties, and AP reporters saw them remove three bodies.

The powerful blast wave scorched nearby buildings, shattered windows and flung a dog into a neighboring yard, resident Halina Sichievka said.

“Now we don’t have anything in our apartment, no windows, no doors, nothing. Nothing at all,” the 28-year-old said.

The Ukrainian air force said some of the weapons used in the attack were Kinzhal hypersonic missiles, which are among the most advanced Russian weapons. They fly at 10 times the speed of sound, making them hard to intercept.

Three electricity substations were damaged or destroyed in two districts of Kyiv, energy company DTEK said.

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AP journalist Samya Kullab in Kyiv contributed to this report.

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Follow AP’s coverage of the war in Ukraine at https://apnews.com/hub/russia-ukraine