Ukraine President Volodymyr Zelensky has said a mass drone strike targeting Russian airbases is Ukraine's "most long-range operation" in more than three years of war.
Mr Zelensky also said agents involved in preparing the drone strikes had been "withdrawn" from Russian territory in time, after Russia's defence ministry said several "participants" had been arrested.
His comments follow a Ukrainian operation in which Russian bombers worth billions of dollars were destroyed in a "large-scale" drone assault on Russian air bases.
This came as both countries are preparing for talks to explore prospects for a ceasefire.
Ukraine said it damaged $7 billion (€6.1bn) worth of Russian aircraft parked at four airbases thousands of kilometres away, with unverified video footage showing aircraft engulfed in flames and black smoke.
A source in the Ukrainian security services said the drones were concealed in the ceilings of shipping containers which were opened to release them for the attack.
The sheds were loaded onto trucks that were driven to the perimeter of the air bases, the official said, adding the roof panels of the sheds were lifted off by a remotely activated mechanism, allowing the drones to fly out and begin their attack.
The security official, who spoke on condition of anonymity, said strikes were conducted on four air bases, and that a total of 41 Russian warplanes were hit.
Reuters was not able to independently verify that assertion.
Unverified video and pictures posted on Russian social media showed Russian strategic bombers on fire at the Belaya air base in the Irkutsk region of Siberia.
Russia said a number of suspects were arrested after confirming that Ukraine launched drone strikes targeting Russian military airfields across five regions, causing several aircraft to catch fire.
The attacks occurred in the Murmansk, Irkutsk, Ivanovo, Ryazan, and Amur regions.
Air defences repelled the assaults in all but two regions, Murmansk and Irkutsk, the Russian defence ministry said.
"In the Murmansk and Irkutsk regions, the launch of FPV drones from an area in close proximity to airfields resulted in several aircraft catching fire," the ministry said.
The fires were extinguished and no casualties were reported. Some individuals involved in the attacks had been detained, the ministry said.
The operation came at a delicate moment three years into Russia's invasion.
Ukraine's President Volodymyr Zelensky said he was sending a delegation to Istanbul led by his Defence Minister Rustem Umerov for talks tomorrow with Russian officials.
Turkey is hosting the meeting, which was spurred by US President Donald Trump's push for a quick deal to end the three-year war.
Mr Zelensky, who previously voiced scepticism about whether Russia was serious in proposing tomorrow's meeting, said he had defined the Ukrainian delegation's position going into it.
Priorities include "a complete and unconditional ceasefire" and the return of prisoners and abducted children, he said on social media.
Russia has rejected previous ceasefire demands.
It said it had formulated its own peace terms but refused to divulge them in advance.
Russian President Vladimir Putin ruled out a Turkish proposal for the countries' leaders to attend the meeting.
Russian news agencies said the Russian delegation was headed to Istanbul today for the talks.
Meanwhile a source in Ukraine's SBU security service said the coordinated attacks inside Russia were "aimed at destroying enemy bombers far from the front".
Rybar, an account on the Telegram message platform that is close to the Russian military, called it a "very heavy blow" for Moscow and pointed to what it called "serious errors" by Russian intelligence.
The SBU source said strikes targeted Russian airbases in the eastern Siberian city of Belaya, in Olenya, in the Arctic near Finland, and in Ivanovo and Dyagilevo, both east of Moscow.
More than 40 aircraft had been hit at the Belaya base and a fire had broken out there, the source said, showing a video in which several aircraft could be seen in flames and black smoke rising.
AFP was not able to independently verify the claims or the video images.
The SBU claimed in a social media post to have hit Russian military planes worth a combined $7 billion (€6.1bn) in a "special operation".
Russia's defence ministry confirmed on Telegram that several of its military aircraft "caught fire", adding that there were no casualties and that several "participants" had been arrested.
Igor Kobzev, Governor of Russia's Irkutsk region, which hosts the targeted Belaya airbase, said it was "the first attack of this sort in Siberia".
He called on the population not to panic and posted an amateur video apparently showing a drone flying in the sky and a large cloud of grey smoke.
Governor of the Murmansk region Andrey Chibis - where the Olenya base was located - also said "enemy drones" were flying overhead and anti-aircraft defences were operating.
The Irkutsk region attack was the first time a drone assault had been mounted by Ukraine so far from the front lines, which are more than 4,300 km (2,670 miles) away.
That is beyond the range of the long-range strike drones or ballistic missiles Ukraine has in its arsenal, so required a special scheme to get the drones close enough to the target.
Photographs shared with Reuters by the Ukrainian security official showed dozens of short-range quadrocopter drones piled up in an industrial facility. The official said these were the same devices used in the attack.
Other images shared by the official showed the wooden sheds with their metal roofing panels removed, and the drones sitting in the cavities between roof beams.
Separate video posted on Russian Telegram channels, which has not been verified by Reuters, appeared to show matching sheds on the back of a truck.
The roof panels can be seen lying on the ground next to the truck, and the video footage shows at least two drones rising out of the top of the sheds and flying off.
Operation 'Spider's Web'
The Russian online media outlet that posted the video, Baza, said in a caption that it was filmed in the district near the Belaya air base.
The Irkutsk region air base hosts Tupolev Tu-22M supersonic long-range strategic bombers, a type of aircraft that has been used to launch missiles against targets in Ukraine.
The operation was code-named "Spider's Web", according to the Ukrainian security official, and was personally overseen by Mr Zelensky and Vasyl Maliuk, head of the SBU domestic intelligence agency.
If confirmed, the strikes would be the most damaging Ukrainian drone attack of the war, and would be a significant setback for Moscow.
The source shared video footage shot from a drone, saying it showed one of the strikes. The images showed several large aircraft, some of which appeared to be Tu-95 strategic bombers, on fire.
Separately, Russian officials said the overnight collapse of two bridges in Russian regions bordering Ukraine that killed seven people were caused by explosions,
Authorities did not say who was behind the explosions, but investigators said a criminal inquiry was under way and it was being treated as "acts of terrorism".
Russia and Ukraine step up the war on eve of peace talks
Russia has been announcing Ukrainian drone attacks on a near-daily basis, usually saying they had all been shot down.
But it was rare for such drone strikes to be reported so deep within its territory.
At the same time, Russia has been carrying out constant attacks on Ukraine.
Ukraine's air force said it was hit by 472 Russian drones and seven missiles overnight, a record since the beginning of the invasion.
In a rare admission of its military losses, the Ukraine army said Russia's "missile strike on the location of one of the training units" had killed a dozen soldiers, most of whom had been in shelters during the attack and wounded more than 60.
The attack led Ukrainian ground forces commander Mykhailo Drapaty to announce his resignation, saying he felt "responsibility" for the soldiers' deaths.
Separately, the Russian army said it had captured another village in Ukraine's northern Sumy region, where Kyiv fears Moscow could mount a fresh ground assault.
Russia claims to have captured several settlements in the region in recent weeks and has massed more than 50,000 soldiers on the other side of the border, according to Mr Zelensky.
Authorities in the region have evacuated more than 200 villages amid intensified shelling.
In Russia, officials said a blast brought down a road bridge in the Bryansk region bordering Ukraine yesterday, derailing a passenger train heading to Moscow and killing seven people.
A separate rail bridge in the neighbouring Kursk region was blown up hours later in the early hours of Sunday, derailing a freight train and injuring the driver.
Authorities did not say who was behind the explosions, but investigators said a criminal inquiry was under way.
