Russia and Ukraine exchanged 205 prisoners of war on Friday, hours after rescue workers ended their search of a destroyed block of flats in Kyiv in which 24 people were killed, including three girls.
Most of the Ukrainian prisoners had been held since 2022, said President Zelensky.
The swap was part of a short-lived ceasefire ending this week with the launch of massive Russian strikes across Ukraine, including a missile attack that reduced 18 flats to rubble. Among the victims was 12-year-old Lyubava Yakovleva, whose father was killed during the war.
Meanwhile, Russian officials said four people, including a child, were killed when Ukrainian drones hit the city of Ryazan, south-east of Moscow.
Images on social media showed damage to two blocks of flats, which Ryazan Governor Pavel Malkov said had been damaged by debris.
Twenty-eight people were hurt in the attack. Ukrainian's drone commander said his forces had hit Ryazan's oil refinery - one of the biggest in Russia.
Rescue services in the Darnytskyi district of south-east Kyiv completed their search of the rubble of a devastated nine-storey apartment block on Friday. Throughout the 28-hour operation residents and soldiers looked on, waiting for news of missing relatives.
Kyiv was marking a day of mourning on Friday for the 24 victims of the strike.
The Nova Poshta postal service said two of its staff, both named Dmytro, had been killed in the attack, one of them alongside his wife. Former hockey player Yuriy Orlov and his 24-year-old girlfriend Maryna Homeniuk, an English teacher, were among the victims. A kindergarten teacher, Svitlana Moskalishyna, was also killed.
The three girls who died were 12, 15 and 15.
Lyubava Yakovleva's elder sister was initially declared missing in the rubble, and it was later reported that she too had been killed. The girls' school said on Facebook that the "scariest face of war is the children's lives it takes".
Among those bringing flowers to the scene was Volodymyr Zelensky, who said pressure had to be brought to bear on a Russia that "deliberately destroys lives and hopes to remain unpunished".
The Ukrainian leader said the building had been practically levelled to the ground by a Russian X-101 cruise missile.
Most of the 18 flats destroyed in Thursday's strike were single-room apartments, or the bathrooms and kitchens of two-room apartments. An entrance to the building was reduced to rubble.
Prime Minister Yulia Svyrydenko praised rescuers for saving the lives of 30 people.
One of the first people to reach the scene of the strike, 18-year-old Ivan, rushed to the block with his father to help rescue the victims.
"A fire was burning, and there was smoke everywhere," the teenager told public broadcaster Suspilne. "I helped a policeman lower a disabled woman [to safety] she couldn't walk and the policeman carried her in his arms."
Zelensky said the Russian missile had been produced in recent weeks. "This means that Russia is still importing the components needed for missile production, as well as the necessary resources and equipment, in circumvention of global sanctions," he posted on social media.
He also paid tribute to the 205 Ukrainians released in the first stage of a planned exchange of 1,000 prisoners on each side.
The deal was brokered by the US and United Arab Emirates and Russia's defence ministry said that 205 Russians were taken to Belarus, where they were being given medical and psychological support.
Zelensky said among those freed were Ukrainians who had fought during the siege of Mariupol in the initial months of Russia's full-scale invasion from February 2022, as well as in several border regions and at the nuclear plant at Chornobyl, previously known during the Soviet era as Chernobyl.
The swap was part of a three-day ceasefire that the two sides had agreed to from 9-11 May, which covered Russia's scaled-down Victory Day parade in Moscow.
The truce, which was marred by a series of violations, was quickly forgotten this week when Russian forces launched their biggest drone onslaught since the start of the war.
Ukrainian officials said 1,410 Russian drones and 56 missiles been launched at Ukrainian cities and communities in just one 24-hour period from 13-14 May.
Although Russia's President Putin had spoken last Saturday of the war "heading to an end", there have been no negotiations since February and there has been little sign of any progress in recent days.
Ukrainian commentators have suggested the escalation in Russian attacks was timed to coincide with US President Trump's trip to Beijing for talks with Chinese President Xi Jinping. Zelensky accused Moscow of trying to "disrupt the overall political atmosphere".
The Kremlin says Vladimir Putin will visit Xi Jinping "really soon", in the aftermath of Xi's talks with Donald Trump.
Spokesman Dmitry Peskov said Putin and Xi would focus on bilateral ties as well as international matters.
