KYIV — A day of mourning is being observed in Ukraine after one of the worst waves of Russian missile strikes in months, with at least 41 people killed and 166 injured.

The main children's hospital in the capital Kyiv was among buildings hit in cities across the country on Monday.

Two people died when a missile flattened part of the Ohmatdyt Children's Hospital - Ukraine's biggest paediatrics facility - and a search for survivors beneath the rubble continued into the early hours of Tuesday.

Elsewhere, the governor of Russia's southern Belgorod region, which borders Ukraine, said four people had died in Ukrainian strikes in the last 24 hours.

On Monday, Russia denied targeting the Kyiv hospital, saying it had been hit by fragments of a Ukrainian air defence missile, while Ukraine said it had found remnants of a Russian cruise missile.

Ukrainian President Volodymyr Zelensky called the attack "brutal" and described his Russian counterpart Vladimir Putin as a "bloody criminal".

NATO is preparing to hold a summit later on Tuesday in Washington, where the military bloc's response to the invasion of Ukraine will be high on the agenda.

World leaders allied to Ukraine have condemned the attack, including the new British Prime Minister, Sir Keir Starmer.

He said "attacking innocent children" was "the most depraved of actions" and promised continued support for Kyiv after the change of government in the UK.

Calling the strikes a "horrific reminder of Russia's brutality", US President Joe Biden said additional support for Ukraine's air defence systems would soon be announced.

Mr Zelensky said that Russia had launched more than 40 missiles on Monday, damaging almost 100 buildings in Kyiv, Dnipro, Kryvyi Rih, Sloviansk and Kramatorsk.

Pictures from the scene of the blast at the Kyiv hospital - which specialises in cancer treatment and organ transplants - showed children hooked up to IV drips sitting outside the damaged facility awaiting evacuation.

Rescue workers and medics dug through the rubble to look for survivors, though it was unclear how many were still trapped.

On Monday afternoon, Kyiv's Mayor Vitali Klitschko said 16 people, including seven children, had been injured in the hospital strike.

He said the attacks across the city were among the worst Kyiv had faced since the beginning of the war, as he ordered flags to be flown at half-mast on Tuesday and cancelled entertainment events.

Following the strike, Ukrainian tennis player Elina Svitolina wore a black ribbon as a mark of respect when she played in the round of 16 at Wimbledon on Monday afternoon.

She fought back tears during her post-match interview, saying: "It wasn't easy to focus today on the match.

"Since the morning it was difficult to read the news. To go on the court is extremely tough."

The UN's human rights monitoring mission in Ukraine has said civilian casualties have been mounting in recent months, as Russia renewed its air campaign.

A recent report said May was the deadliest month for civilian deaths in almost a year.

On Tuesday morning, the governor of the Rostov region in southern Russia said a fire had broken out at a powerplant after Ukraine launched "tens" of drones. — BBC