By Anastasiia Malenko and Olena Harmash

KYIV (Reuters) -Russia flattened a section of an apartment block in Kyiv on Tuesday, its deadliest attack on the Ukrainian capital this year, as a huge barrage of hundreds of drones and dozens of missiles killed at least 18 people and wounded 151.

Ukrainian officials declared a day of mourning on Wednesday for victims of what President Volodymyr Zelenskiy described as one of the most horrific attacks on the capital during the war. Authorities said 16 were killed in Kyiv and two in Odesa.

"Such attacks are pure terrorism. The whole world, the United States, and Europe must finally respond as a civilized society responds to terrorists. (Russian President Vladimir) Putin does this solely because he can afford to continue the war."

Zelenskiy said Russian forces had sent 440 drones and fired 32 missiles at Ukraine.

The Ukrainian president told the summit in Canada of the Group of Seven industrialised nations that the attack proved once again the need for better air defences.

"This is a matter of life and death," he said in comments posted on Telegram. "We must continue to receive air defence systems and missiles and move to localising production in Ukraine."

The United Nations Human Rights Monitoring Mission in Ukraine said it was the most deadly attack this year on Kyiv and underscored the dangers of using such weaponry in major cities.

"Last night's attack exemplifies the grave threat posed by the tactic of deploying missiles and large numbers of drones simultaneously into populated areas, which leads to civilian casualties, and profound suffering," Danielle Bell, Head of HRMMU, said in a statement.

Russia's defence ministry said it had used air, land and sea-based missiles and drones to strike "objects of the military-industrial complex of Ukraine" in the Kyiv region and southern Zaporizhzhia province.

Ukrainian officials said about 27 locations in the capital were hit during several waves of attacks throughout the night, that damaged residential buildings, educational institutions and critical infrastructure.

A missile struck a nine-storey residential building in Kyiv's Solomianskyi district, wiping out a whole section of it, which was flattened into a pile of debris.

Emergency workers combed through the rubble and doused flames with hoses. They used a crane to lower a wounded elderly woman in a stretcher out of the window of a flat in an adjacent section of the building.

'SIMPLY HORRIFIC'

"I have never seen anything like this before. It is simply horrific. When they started pulling people out, and everyone was cut up, elderly people and children... I do not know how long they can continue to torment us ordinary people," said Viktoriia Vovchenko, 57, who lives nearby.

Ukraine's State Emergency Services said operations proceeded throughout Tuesday, with three bodies pulled from rubble late in the evening, bringing the casualty toll in Kyiv to 16 dead and 134 injured. Kyiv Mayor Vitali Klitschko said the dead included a 62-year-old U.S. citizen, who died from shrapnel wounds.

Two people were killed and 17 injured in the Black Sea port of Odesa.

Ukraine has launched drones deep into Russia, although its attacks have not caused similar damage to civilian targets.

Russia's Defence Ministry said it had intercepted and destroyed 147 Ukrainian drones over Russian territory, including the Moscow region, overnight.

Moscow has used drones and missiles to hit Ukrainian cities far from the front throughout the war. Its attacks have become more deadly in recent weeks, even as the sides have held their first peace talks in more than three years.

Oleksandr Kovalenko, a Ukrainian military analyst, said that since the start of June the Russians had turned to a new tactic of concentrating drone and missile strikes on a single city at a time to overwhelm Ukraine's air defences.

"It is a new challenge that we need to adapt to as soon as possible," he said.

Russia's full-scale invasion is now in its fourth year, and the hostilities have heated up in recent weeks as Kyiv and Moscow failed to reach any agreement during two rounds of peace talks in Istanbul.

Russian troops are pressing on with a grinding advance in eastern Ukraine and have opened a new front in the Sumy region in the northeast, despite calls for a ceasefire from U.S. President Donald Trump, who promised to end the war quickly.

Zelenskiy was at the G7 summit to garner more support for tighter sanctions on Russia and continued military aid for Ukraine.

He had hoped to meet Trump, but the U.S. president left the summit a day early, with the White House citing the situation in the Middle East.

Trump has reoriented U.S. policy away from supporting Kyiv toward accepting Moscow's justifications for its invasion, and has so far resisted calls from European allies to impose tighter sanctions on Moscow for rejecting his calls for a ceasefire.

(Additional reporting by Gleb Garanich, Sergiy Karazy, Valentyn Ogirenko and by Olivia le Poidevin in GenevaWriting by Olena Harmash and by Peter GraffEditing by Gareth Jones, Ron Popeski and David Gregorio)