An unknown number of people have been trapped under rubble after Ukraine’s largest children’s hospital was hit during a daylight Russian missile barrage that the authorities said had killed at least 29 people across the country.

Volodymyr Zelenskiy vowed retaliation as he said Kyiv’s Okhmatdyt hospital, the main treatment centre in the country for children with cancer, had taken a direct missile hit. The strike was part of one of the heaviest attacks on the capital since Russia’s February 2022 invasion.

“Russia cannot be unaware of where its missiles are landing and must be held fully accountable for all its crimes: against individuals, against children and against humanity as a whole” Zelenskiy posted on the Telegram messaging app.

Ihor Klymenko, the interior minister, said five people had been confirmed dead in the Kyiv hospital attack and at least four more had been injured.

The strike largely destroyed the hospital’s toxicology ward. 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 remained trapped under the rubble.

“We are extracting whoever we can. We don’t know the number of people trapped there,” said the health minister, Viktor Liashko, outside the hospital.

Maria Soloshenko, 21, a nurse in the toxicology ward, said hospital staff had been in the process of moving the children to a bomb shelter when the explosion occurred. “There was immediate panic when the strike hit,” she said, her gloves covered in blood.