Ukraine searches children's hospital ruins as NATO countries condemn Russian strike

First responders spent the night in Kyiv digging through the rubble at the children's hospital and other sites struck by Russian missiles on Monday, Ukrainian President Volodymyr Zelenskyy said.

"The rescue operation after yesterday's Russian missile attack continues," he said on social media on Tuesday. "As of now, 38 people have been reported dead, including four children. My condolences to all the families and friends affected by this tragedy."

At least 94 children, including eight who were injured, were transferred to other hospitals following the missile strike on the Okhmatdyt Children's Hospital in Kyiv, Mayor Vitali Klitschko said on the messaging app Telegram.

The wide-ranging aerial assault on Monday came on the eve of a NATO summit in Washington, D.C. As rescue work in Kyiv continued, officials with NATO countries and other Ukrainian allies issued scathing remarks criticizing Russia for its "horrendous" and "barbaric" strikes.

Secretary General Jens Stoltenberg condemned Russia's action, describing the strikes as "horrendous missile attacks against Ukrainian cities, killing innocent civilians including children."

NATO members will make decisions during the Washington summit about how to strengthen the alliance's support for Ukraine , according to a press release from Stoltenberg's office.

President Joe Biden said on Monday that the fatal missile strikes in Ukraine were "a horrific reminder of Russia's brutality." He added that the world must continue to stand with Ukraine ahead of this week's NATO summit, where the ongoing war will be a main topic of discussion.

Biden said that he will make clear to Zelenskyy that "support for Ukraine is unshakeable." NATO leaders would also be announcing "new measures to strengthen Ukraine's air defenses," he said.

"My thoughts are with all the victims and their loved ones," David Lammy, the U.K. foreign secretary, said in a statement. "The U.K.'s support to Ukraine is iron-clad. We must hold those responsible for Putin's illegal war to account."

Stéphane Séjourné, the French foreign minister, condemned Russia's missile strikes, calling them "barbaric" and "war crimes." He called for Ukrainian allies to strengthen their support for the under-seige nation.

"Russia must not win this war," Séjourné said in French on social media.

The Security Service of Ukraine said it had opened a criminal investigation into the attack, which it also called a "war crime." The service said a Russian Kh-101 missile struck the site, according to its preliminary findings.

Martin Jaeger, the German ambassador to Ukraine, said his team visited the hospital site and saw young cancer and dialysis patients "sitting on the sidewalk with their mothers."

"This is a war against the civilian population," he wrote in German on social media. "This is what Russia's readiness for negotiations and its desire for peace looks like."

The German prime minister added in a statement that his country "stands unwaveringly by the side of the Ukrainian people. Especially in these difficult times."

More than 400 people were busy overnight on rescue operations at the Kyiv children's hospital and elsewhere, Zelenskyy said on Tuesday.

He said four of the 38 people killed had been children. At least 190 people were being treated for injuries across the country, he said.

"We continue our work to increase the protection of our cities and communities from Russian terror," Zelenskyy said. "There will be decisions. The world has the necessary strength for this."

ABC News' Michelle Stoddart contributed to this report.