An oil storage depot was set on fire in a Russian city just north of Ukraine after what Russia described as an attack by two Ukrainian helicopters.
A video shared on Twitter showed a blaze near apartment blocks in Belgorod, 40km (25 miles) from the border.
Some clips appeared to show rockets hitting the oil depot.
Ukraine's top security official, however, denied Ukrainian forces were behind the attack.
"For some reason, they are saying we are behind it. This does not correspond to reality," security council secretary Oleksiy Danilov said.
Ukrainian aircraft have not struck targets in Russia previously.
Yet Belgorod governor Vyacheslav Gladkov accused Ukraine of launching the attack, and later Russian defence ministry spokesman Igor Konashenkov gave details.
He said that at around 05:00 Moscow time (02:00 GMT) two Ukrainian Mi-24 helicopters entered Russian airspace at extremely low altitude and "launched a missile attack on a civilian oil storage facility" on the outskirts of Belgorod. Some storage tanks were damaged and caught fire, he said.
"The oil storage facility has nothing to do with the Russian armed forces," he said.
President Vladimir Putin's spokesman said the incident "cannot be perceived as creating comfortable conditions for continuing the talks" with Kyiv. So far those peace talks have made little progress.
The spokesman, Dmitry Peskov, said Russia was now trying to reorganise the fuel supply chain to prevent disruption of Belgorod's energy supplies.
The city of 370,000 lies just north of Ukraine's second city Kharkiv, which has been heavily shelled by Russian artillery and remains surrounded by Russian forces.
Governor Gladkov said in a Telegram message that nobody was killed at the oil depot, which is run by Russian state oil firm Rosneft. He said emergency workers were trying to contain the fire and there was "no threat" to residents. The emergencies ministry posted video of the blaze on Telegram.
Russia's Interfax news agency reported that residents nearby were evacuated and two people were injured at the depot. It said eight fuel tanks were on fire and nearly 200 firefighters were on the scene.
Queues of cars later formed at local petrol stations, but Mr Gladkov said Belgorod's fuel supplies were still plentiful.
Russia's RIA Novosti news agency says the blaze in three of the tanks has been extinguished, but there is still a risk of the fire spreading.
On 29 March several explosions were reported at an ammunition depot near Belgorod.
Commenting on the oil depot blaze, Ukrainian defence ministry spokesman Oleksandr Motuzyanyk said Russian "aggression" was being resisted "on the territory of Ukraine".
"It does not mean that Ukraine has to bear responsibility for all those catastrophes and all those events that happen on the territory of the Russian Federation. This is not the first time we have seen such accusations. So, dear friends, I will neither confirm nor deny this information," he said.
Ukraine is yet to claim responsibility for this attack, but if it were confirmed it would be the first time that Ukrainian aircraft have flown into Russian airspace to hit a target. bringing the war home to Russia.
Ukrainian helicopter pilots have plenty of experience of flying low and fast to avoid being detected by military radar and air defence systems. They've been doing exactly that in the Donbas region in eastern Ukraine for years. I experienced and witnessed the extraordinary skills of the Ukrainian military pilots in 2018 - flying barely metres above the tree line and telegraph poles.
But if these unconfirmed reports are correct - flying at night, well into Russian territory, to launch an attack on an enemy fuel depot would have required extraordinary bravery - as well as finely-honed flying skills.
Low-flying helicopters are still vulnerable to short-range air defence systems. Flying at night would have lessened that risk, but heightened the danger of hitting an object near the ground.
The Mi-24, or Hind helicopter, is known as the "flying tank". Its rockets would have been the weapon deployed to target the oil depot in Belgorod, Russia.
This alleged attack alone will not dramatically alter the battle. But it could show Ukraine has managed to keep its air force functioning, and give a huge boost to the morale of Ukraine's military.
A well-known Ukrainian journalist in Kyiv, Yury Butusov, posted on Facebook that it was a missile attack at 05:50 local time (02:50 GMT) by "two Ukrainian Mi-24 combat helicopters, which flew from Ukraine to Russian Belgorod over low altitudes, inconspicuous for Russian anti-aircraft defence".
Vladimir Soloviev, host of a staunchly pro-Kremlin current affairs programme on Russian state TV, asked in a tweet: "Question. Who is in charge of the Belgorod air defences? Have the bases of the helicopters that delivered this strike been destroyed? When will a safe zone be set up in Belgorod region?"