Russia is blaming a Ukrainian strike after an entire section of an apartment block collapsed following an explosion in Belgorod.
CCTV from the scene shows a large blast near the base of the 10-floor structure and then the building falling in.
At least 19 people have been injured and there are reports of fatalities and people being trapped in the rubble.
The Russian city is near the border with Kharkiv in Ukraine, where Moscow's troops launched an offensive on Friday.
The Belgorod region has often been targeted by Ukrainian forces since the start of Russia's full-scale invasion of Ukraine in 2022, but there has been no word from Kyiv on the latest incident.
Previously Ukrainian officials have said cross-border strikes, including those on Belgorod, do not target civilians.
'He could not escape'
But the regional governor of Belgorod, Vyacheslav Gladkov, accused Ukraine of bombarding the region, describing the cause of the explosion as a Ukrainian shell.
Pictures from the scene show rescuers removing debris by hand in a search for survivors. Surrounding apartments have been evacuated due to fears of the main building collapsing further.
One resident told Russian state news agency RIA Novosti her husband was in their bedroom in the building when the blast went off and was injured in the head and face. "He did not have the time" to escape, she said.
A spokesperson for the Russian Investigative Committee said a criminal case has been launched.
The incident comes as fighting continues just across the border in north-eastern Ukraine, where Russian forces have made incursions into the Kharkiv region.
The defence ministry in Moscow claims its forces have captured a number of villages there, while Kyiv says it has been carrying out counter-attacks.
