Meanwhile, Russia pounded Ukraine with missiles and drones on Sunday, a day before the two sides meet for a new round of direct talks in Istanbul.

The official, who spoke on condition of anonymity to disclose operational details, said the attack took more than 1½ years to execute and was personally supervised by Ukrainian President Volodymyr Zelenskyy.

The operation saw drones transported in containers carried by trucks deep into Russian territory, he said. The drones hit airfields including the Belaya air base in Russia’s Irkutsk region, more than 4,000 kilometres from Ukraine.

It was the first time that a Ukrainian drone had been seen in the region, local Governor Igor Kobzeva said, stressing that it did not present a threat to civilians.

The attack was disclosed on the same day as Zelenskyy said Ukraine would send a delegation to Istanbul for a new round of direct peace talks with Russia on Monday.

In a statement on Telegram, Zelenskyy said that Defence Minister Rustem Umerov will lead the Ukrainian delegation.

“We are doing everything to protect our independence, our state and our people,” Zelenskyy said.

Ukrainian officials had previously called on the Kremlin to provide a promised memorandum setting out its position on ending the war before the meeting takes place. Moscow had said it would share its memorandum during the talks.

Russian strike hits an army unit

Russia on Sunday launched the biggest number of drones — 472 — on Ukraine since the full-scale invasion in February 2022, Ukraine’s air force said.

Russian forces also launched seven missiles alongside the barrage of drones, said Yuriy Ignat, head of communications for the air force.

Earlier on Sunday, Ukraine’s army said at least 12 Ukrainian service members were killed and more than 60 were injured in a Russian missile strike on an army training unit.

The strike occurred at 12.50pm (7.50pm AEST), the statement said, emphasising that no formations or mass gatherings of personnel were being held at the time.

An investigative commission was created to uncover the circumstances around the attack that led to such a loss in personnel, the statement said.

The training unit is located to the rear of the 1000-kilometre active front line, where Russian reconnaissance and strike drones are able to strike.

Ukraine’s forces suffer from manpower shortages and take extra precautions to avoid mass gatherings as the skies across the front line are saturated with Russian drones looking for targets.

“If it is established that the actions or inaction of officials led to the death or injury of servicemen, those responsible will be held strictly accountable,” the Ukrainian Ground Forces’ statement said.

Russia’s Ministry of Defence said on Sunday that it had taken control of the village of Oleksiivka in Ukraine’s northern Sumy region.

Ukrainian authorities in Sumy ordered mandatory evacuations in 11 more settlements Saturday as Russian forces make steady gains in the area.

Speaking on Saturday, Ukraine’s top army chief, Oleksandr Syrskyi, said that Russian forces were focusing their main offensive efforts on Pokrovsk, Toretsk and Lyman in the Donetsk region, as well as the Sumy border area.