Two blasts have been reported in a Russian border city, the latest in a series of attacks on key sites within Russia's borders, two eyewitnesses have reported.
The blasts were heard in the southern part of the Belgorod. It was not immediately clear what caused them and whether there were any casualties or damage.
Russia has in recent days reported what it says are a series of attacks by Ukrainian forces in Belgorod and other southern regions which border Ukraine, and has warned that such attacks raise a risk of significant escalation.
On Wednesday the Russian government said an ammunition depot in the south of the country had been attacked by Ukraine.
The governor of the region of Wednesday's attack said no civilians had been injured.
Ukraine has not directly accepted responsibility but has described the incidents as payback and "karma" for Russia, nine weeks after it invaded its neighbour.
Ukrainian presidential adviser Mykhailo Podolyak said it was natural that Russian regions where fuel and weapons are stored were learning about "demilitarisation" but did not claim responsibility for the attack.
The use of that word was a pointed reference to Moscow's stated objective for the nine-week-old war in Ukraine, which it calls a special military operation to disarm and "denazify" its neighbour.
"If you (Russians) decide to massively attack another country, massively kill everyone there, massively crush peaceful people with tanks, and use warehouses in your regions to enable the killings, then sooner or later the debts will have to be repaid," Podolyak said.
He added it was not possible to "sit out" the Russian invasion. "And therefore, the disarmament of the Belgorod and Voronezh killers' warehouses is an absolutely natural process. Karma is a cruel thing," he said.
Earlier this month, Russia accused Ukraine of attacking a fuel depot in Belgorod with helicopters, which a top Kyiv security official denied, and opening fire on several villages in the province.
The incidents have exposed Russian vulnerabilities in areas close to Ukraine that are vital to its military logistics chains.
There have also been reports of attempted attacks on the Ukraine border region of Kursk and in the city of Voronezh.
The fact Ukraine now appears to be capable of striking Russia will cause further headaches for Moscow, which convinced its population the war would be won quickly.
The UK has backed up Ukraine's right to attack Russian soil with junior defence minister James Heappey saying on Tuesday it was "completely legitimate" for Kyiv to launch the strikes.
He added that the weapons the West - including the UK - is giving to Ukraine "have the range to be used over the border" into Russia.
The continued supply of western made weapons to Ukraine has infuriated the Kremlin.
On Monday, Russian defence minister Sergei Lavrov said there was a "real danger" of the conflict slipping into a third world war, and said Russia viewed Nato as being “in essence” engaged in a proxy war as sovereign nations are supplying Ukraine with weapons.