Russia has accused Ukraine of firing missiles armed with cluster bombs at residential areas in Belgorod, killing at least three civilians.
Ukrainian officials have not responded to the Russian accusation. If it is confirmed, it would mark an escalation of the war, although the West has previously accused Russia of "false flag" attacks aimed at spreading hostilities.
"All necessary medical assistance is being provided," Vyacheslav Gladkov, the Belgorod region governor, told Russian media after touring the alleged attack site.
"At the moment, there is information about three dead."
Russian media published a video of destroyed houses and another video of an explosion on a residential block as evidence of the attack, but these could not be independently verified.
Belgorod lies just across the border from Kharkiv in northern Ukraine and has been used as a major resupply point for Russian forces fighting both on the Kharkiv frontlines and also the further east in Donbas.
In April, two Ukrainian helicopters blew up a fuel depot in Belgorod in a dawn attack, but the city has not previously been the target of a Ukrainian missile attack.
The Russian Ministry of Defence said that its missile defence system had knocked out the three Tochka-U missiles fired from Ukraine, but that the debris hit residential houses.
"Tonight, between 3:00 and 3:30 Moscow time, the Kyiv regime carried out a deliberate strike with Tochka-U ballistic missiles with cluster munitions and Tu-143 Reis drones on residential areas of Belgorod and Kursk, where there are no military facilities," a spokesman for Russia's Ministry of Defence said.
The Tochka-U missile is Soviet-manufactured and designed to kill infantry soldiers.
It deploys in a similar way to a cluster bomb by exploding above the ground into hundreds of fragments which slice through flesh, killing and maiming as many people as possible.
In April, Russian forces fired a Tochka-U missile at a train station in Kramatorsk, Donbas, crowded with refugees.
At least 59 people were killed and 110 injured. Ukraine has begged its Western allies to supply it with more artillery pieces for its battles in Donbas.
Britain and the US have agreed to send long-range artillery to Ukraine, but only on the condition that they are not used against targets inside Russia.
Russia also said that villages along the border with Ukraine in its Kursk region were also shelled. It has previously accused Ukraine of shelling villages along its border without producing conclusive evidence.
Again, Ukraine has not commented on these Russian accusations.
It did claim responsibility, though, for a missile attack on a Russian army base near the city of Melitopol, southern Ukraine, which Russia seized in March.
The ousted Ukrainian mayor of Melitopol, Ivan Fedorov, said that the missile attack had destroyed a Russian army base. “We fired over 30 shots on military bases, and only military targets," he said.
The Kremlin has started to incorporate the Zaporizhzhia region and neighbouring Kherson into Russia. It has introduced the rouble currency, planted Russian flags across public places and imposed the Russian curriculum on schools.
This has triggered a Ukrainian reaction, ranging from partisan assassination attacks, to pledges to retake the occupied southern regions and missiles strikes.