Joe Biden has called one of the heaviest Russian airstrikes on Ukraine since the war began “a horrific reminder of Russia’s brutality”, amid widespread international revulsion at Monday’s attacks and as Nato leaders gathered to announce new measures to strengthen Ukraine’s air defences.

The government of Volodymyr Zelenskiy declared Tuesday a day of mourning after at least 37 civilians were killed in a series of attacks where targets included Ukraine’s largest children’s hospital, leaving an unknown number trapped under the rubble in Kyiv.

“It is critical that the world continues to stand with Ukraine at this important moment and that we not ignore Russian aggression,” the US president said in a White House statement, adding “we will be announcing new measures to strengthen Ukraine’s air defences to help protect their cities and civilians.”

The president’s statement came on the eve of a Nato summit in Washington which marks the 75th anniversary of the transatlantic alliance and which will bring together President Volodymyr Zelenskiy and leaders of countries that have provided Kyiv with tens of billions of dollars in military aid.

Zelenskiy said more than 100 buildings were damaged in Monday’s attack, including the children’s hospital and a maternity centre in Kyiv. “The Russian terrorists must answer for this,” he said, adding “being concerned does not stop terror. Condolences are not a weapon.”

Images beamed around the world showed parents holding babies in the streets outside Kyiv’s Okhmatdyt hospital, dazed and sobbing after the rare daylight aerial attack. Windows had been smashed and panels ripped off, and hundreds of Kyiv residents were helping to clear debris.

The strike largely destroyed the children’s hospital toxicology ward, where patients with severe kidney issues were being treated. Hundreds of rescue workers and volunteers joined the effort to clear the debris and search for survivors. Officials and emergency staff said it was not immediately clear how many doctors and patients – dead or alive – remained trapped under the rubble.

Zelenskiy, who was in Poland at the time of the attack before heading to Washington DC, put the death toll across the country at 37, including three children, with more than 170 injured. A Reuters tally of casualties from the sites of attacks in different regions totalled at least 41.

The government called a day of mourning on Tuesday for what is one of the worst air attacks of the war, adding that Monday’s strikes showed the urgent need to upgrade its air defences.

Zelenskiy, addressing a news conference in Warsaw on Monday alongside Polish prime minister Donald Tusk, called on Kyiv’s western allies to give a firm response to the attack.

“We will retaliate against these people, we will deliver a powerful response from our side to Russia, for sure. The question to our partners is: can they respond?” Zelenskiy said.

Zelenskiy has for months said his country does not have enough air defence systems and has requested at least seven more Patriot batteries in addition to those already donated by the United States, Germany and the Netherlands. Russia has exploited the gaps in Ukraine’s air defences to carry out devastating strikes on civilians and infrastructure, as well as to pummel Kyiv’s troops on the frontlines.

Observers expect Nato members to pledge of at least four additional Patriot missile batteries to Ukraine at the conclusion of this week’s summit.

The package put forward by Nato countries has been presented as “historic” and is an widely seen as an attempt to “futureproof” continued aid to Ukraine – but it may not fully satisfy Kyiv.

The UN security council is set to meet on Tuesday at the request of Britain, France, Ecuador, Slovenia and the US.

In response to Monday’s attack, British prime minister Keir Starmer condemned “attacking innocent children” as the “most depraved of actions”, while the Italian foreign minister, Antonio Tajani, called the missile strike a “war crime”.

A spokesperson for António Guterres, the UN secretary general, said he strongly condemned the “particularly shocking” strikes against the children’s hospital and another medical facility.

UN rights chief Volker Türk condemned the Russian strikes as “abominable”. France’s foreign ministry called the bombardment of a children’s hospital “barbaric” and Canadian prime minister Justin Trudeau described the attack as “abhorrent”.

Russia, which has targeted civilian infrastructure throughout the war, denied responsibility for deaths on Monday. In a statement, the defence ministry attributed the incident, without directly referencing the hospital blast, to Ukrainian anti-aircraft missiles, despite visual evidence that appeared to point to a Russian strike.

Andriy Yermak, the head of the Ukrainian president’s office, shared an image on X that appeared to show a Russian missile over Kyiv moments before it struck a hospital, identifying it as a Kh-101 cruise missile. Ukraine’s Security Service said it found wreckage from the cruise missile, which flies low to avoid detection by radar, at the site.

Reuters and Agence France-Presse contributed to this report