Ukrainian President Volodymyr Zelenskyy vowed on Thursday to retaliate for Russia’s strikes on Kyiv that killed at least 21 people and wounded dozens, as he visited an apartment block partially destroyed in the attack.Asked by reporters whether Ukraine would retaliate, Zelensky replied: “Definitely.”Kyiv’s mayor called it the largest ever barrage on the capital with the air force saying Russia had launched 496 drones and 74 missiles, including hard-to-intercept ballistic projectiles.It said it shot down 48 of the missiles and 476 drones.In Moscow, the Kremlin vowed to further ramp up the “pressure” on Kyiv after the strike, sticking to its no-compromise rhetoric as rescuers in Kyiv scoured the rubble for survivors.Russia has routinely launched waves of missiles and drones at Ukrainian cities during its more than four-year invasion, which has become Europe’s deadliest conflict since World War II.In the morning, locals stood on the rubble of destroyed apartment blocks ripped apart by the barrage, as smoke poured over the Kyiv skyline.At one spot, a mother cried as she embraced her son in front of the smouldering debris.Blasts started echoing out late on Wednesday, lasting into the early hours of Thursday as Russian missiles and drones rained down on residential areas in the city centre.Mayor Vitali Klitschko described it as the “enemy’s most massive attack on the capital,” without elaborating.The state emergency services said at least 21 people were killed and 85 were wounded, including two children.The Ukrainian branch of the Red Cross said that its key warehouse was “destroyed” in the Russian strikes, with around $2 million (€1.7 million) worth of humanitarian aid lost.”The Ukrainian Red Cross humanitarian warehouse has been destroyed as a result of a large-scale attack on Kyiv,” the charity said on Facebook, adding: “In total, 320,000 units of humanitarian relief items and equipment, with an estimated value of more than UAH 79 million have been lost.”Meanwhile, the Ukrainian publishing house BookChef Publishing, whose portfolio includes books by George Orwell and Barack Obama, said it had lost around 800,000 books which had been reduced to ash in the aerial bombardment.Some 52,000 people, including 4,500 children packed into underground stations to protect from the barrage, according to the Kyiv metro.Others hunkered down in basements or corridors through the night as blasts shook buildings across the city.Kyiv urged its allies to send more air defence.”Air defence supplies for Ukraine are an absolute and critical priority,” Zelenskyy said in a post on Facebook.”We also very much count on a decision by the United States regarding licences for Patriots.”Ukraine is seeking to manufacture munitions for the US-made missile interceptor system, one of its only ways of defending against Russian ballistic missiles, although defence experts say it will take time to set up production domestically.Zelenskyy cuts short visitThe EU’s top diplomat Kaja Kallas said she would propose new sanctions on Moscow following the attack.But the Kremlin showed no signs it would back down, more than four years into an invasion that has killed hundreds of thousands.The attack came hours after Zelenskyy cut short a visit to Dublin on Wednesday, citing intelligence reports of an impending Russian strike.Zelensky said Russian President Vladimir Putin “has been preparing this massive strike against Ukraine for some time now.”Ukraine has stepped up long-range drone attacks inside Russia in recent weeks, targeting energy infrastructure and military targets.Russian officials have reported repeated strikes in border regions, while Moscow has said its air defences have intercepted hundreds of drones from Ukraine in recent days.US efforts to broker an end to the conflict have so far failed.