At least 35 people were killed across Ukraine on Monday, as Russia launched a barrage of more than 40 missiles in a "massive" attack, Ukrainian officials said.

At least five cities fell under attack including the capital of Kyiv. Okhmatdyt Children's Hospital in Kyiv -- one of the most well-respected children's hospitals in the country -- was damaged, along with residential areas and other infrastructure, President Volodymyr Zelenskyy said on social media.

At least two people were killed in the attack on the hospital, including one female physician. At least seven children were injured, according to officials. Zelenskyy said Monday that doctors, hospital staff and first responders were digging through the debris.

"And the entire world must use all its determination to finally put an end to the Russian strikes. Killing is what Putin brings," Zelenskyy said. "Only together can we bring real peace and security."

The attack, which is among the largest Russian strikes in months, comes a day before U.S. President Joe Biden begins a three-day NATO summit in Washington, where the group is expected to discuss support for Ukraine.