Russia sent a wave of combat drones to attack Ukraine late on Sunday evening, the Ukrainian air defence reported, saying swarms were coming from different directions.
The targets were not initially visible as they constantly changed their flight directions, a tactic the Russian military is using to try and overload the Ukrainian air defence.
Moscow is trying to wear down the Ukrainian population with a constant stream of drone and rocket attacks, with many targeting Ukraine's energy infrastructure to disrupt the power supply to knock out water and district heating.
Earlier, Russian President Vladimir Putin threatened retaliation after a Ukrainian drone attack on the Russian city of Kazan.
"Whoever tries to destroy something in our country will be confronted with many times the destruction in theirs and will regret what they tried to do in our country," he said in a televised video conference.
On Saturday morning, six Ukrainian drones crashed into residential buildings in Kazan, with another hitting an industrial site. There were no casualties, according to official reports.
Kazan is located more than 1,000 kilometres from the Ukrainian border. Although Ukraine has not claimed responsibility for the attack, it is said to be a response to a Russian missile attack on Kiev.
Earlier on Sunday, a fuel depot in the western Russian region of Oryol caught fire after a Ukrainian drone attack, regional governor Andrey Klychkov posted on the Telegram platform.
"Thanks to quick reactions, it was luckily possible to prevent further consequences of the attack - the fire was quickly localised and has now been fully extinguished," Klychkov posted. He said there were no casualties or serious damage.
According to Klychkov, Russian air defence shot down around 20 drones that were primarily targeting fuel and energy supply facilities.
The Russian Ministry of Defence reported the downing of an overall 42 drones in the regions of Oryol, Rostov, Bryansk, Kursk and Krasnodar. The information cannot be independently verified.
The fuel depot near the village of Stalnoi Kon had been attacked a week ago. Ukraine's military justified the attack by stating that the reservoirs serve the Russian army. The depot also supplies the Druzhba oil pipeline to Europe, however.
Russian troops have again been executing Ukrainian prisoners of war, Ukrainian human rights ombudsman Dmytro Lubinets posted on Telegram.
A video showed five Ukrainian prisoners being gunned down by Russians after they were captured, he said.
"Russian war criminals who shoot Ukrainian prisoners of war must be brought before an international tribunal and receive the harshest punishment that the law provides for," Lubinets posted.
Killing prisoners of war is a war crime, and repeated allegations have been made against Russian forces.
In an interview conducted in mid-December, Lubinets put the number of confirmed cases since the start of the war at 177, with the number rising rapidly.
There had been 109 executions this year alone, the ombudsman said.
