KYIV, Ukraine — Ukraine’s capital came under a large-scale Russian drone and missile attack early Saturday with explosions and machine gun fire heard throughout the city, forcing many Kyiv residents to take shelter in underground subway stations.

The nighttime Russian attack came hours after Russia and Ukraine began a major prisoner exchange, swapping hundreds of soldiers and civilians in the first phase of an exchange that was agreed on by the two sides at a meeting in Istanbul last week. The agreement was a moment of cooperation in otherwise failed efforts to reach a ceasefire in the 3-year-old war.

The debris of intercepted missiles and drones fell in at least four city districts of the Ukrainian capital early Saturday, acting head of Kyiv military administration, Tymur Tkachenko, wrote on Telegram. According to Tkachenko, six people required medical care after the attack, and two fires were sparked in the Solomianskyi district of Kyiv.

Before the attack, city mayor Vitalii Klitschko warned Kyiv residents of more than 20 Russian strike drones heading towards Kyiv. As the attack continued, he said drone debris fell on a shopping mall and a residential building in Obolon district of Kyiv. Emergency services were headed to the site, Klitschko said.

The prisoner swap Friday was the first phase of a complicated deal involving the exchange of 1,000 prisoners from each side.

President Volodymyr Zelenskyy said the first phase brought home 390 Ukrainians, with further releases expected over the weekend that will make it the largest swap of the war. Russia’s Defense Ministry said it received the same number from Ukraine.

The swap took place at the border with Belarus in northern Ukraine, according to a Ukrainian official who spoke on condition of anonymity because he was not authorized to speak publicly.

The released Russians were taken to Belarus for medical treatment, the Russian Defense Ministry said.

As the freed men entered the medical facility Friday, people holding signs and photos of their relatives shouted names or brigade numbers, seeking any news of a loved one.

“Vanya!” cried Nataliia Mosych, among the gathered relatives, “My husband!”

The exchange, the latest of dozens of swaps since the war began and the biggest involving Ukrainian civilians at one time, didn’t herald any halt in fighting.

Battles continued along the roughly 1,000-kilometer (620-mile) front line, where tens of thousands of soldiers have been killed, and neither country has relented in its deep strikes.

After the May 16 Istanbul meeting, Turkish Foreign Minister Hakan Fidan called the prisoner swap a “confidence-building measure” and said the parties had agreed in principle to meet again.

But Kremlin spokesman Dmitry Peskov said Friday that there has been no agreement yet on the venue for the next round of talks to end the fighting as diplomatic maneuvering continued.

Russian Foreign Minister Sergey Lavrov said Friday night that Moscow would give Ukraine a draft document outlining its conditions for a “sustainable, long-term, comprehensive” peace agreement once the ongoing prisoner exchange had finished.

European leaders have accused Russian President Vladimir Putin of dragging his feet in peace efforts while he tries to press his larger army’s battlefield initiative and capture more Ukrainian land.

The Istanbul meeting revealed that both sides remained far apart on key conditions for ending the fighting. One such condition for Ukraine, backed by its Western allies, is a temporary ceasefire as a first step toward a peaceful settlement.

Russia’s Defense Ministry said it had shot down 788 Ukrainian drones away from the battlefield between May 20 and May 23.

Ukraine’s air force said Russia fired 175 Shahed and decoy drones, as well as a ballistic missile since late Thursday.