Russian President Vladimir Putin on Sunday vowed to bring more "destruction" to Ukraine in retaliation for a drone attack on the central Russian city of Kazan a day earlier.
Russia accused Ukraine of a "massive" drone attack that hit a luxury apartment block in the city, some 1,000 kilometres (620 miles) from the frontier.
Videos on Russian social media networks showed drones hitting a high-rise glass building and setting off fireballs, though there were no reported casualties as a result of the strike.
"Whoever, and however much they try to destroy, they will face many times more destruction themselves and will regret what they are trying to do in our country," Putin said during a televised government meeting on Sunday.
Putin was addressing the local leader of Tatarstan, the region where Kazan is located, in a road-opening ceremony via video link.
The strike on Kazan was the latest in a series of escalating aerial attacks in the nearly three-year conflict.
Ukraine has not commented on the strike.
Putin has previously threatened to target the centre of Kyiv with a hypersonic ballistic missile in response to Ukrainian attacks on Russian territory.
And the defence ministry has called Russian strikes on Ukrainian energy facilities over recent weeks retaliatory hits for Kyiv using Western-supplied missiles to hit Russian air bases and arms factories.
The latest threat comes as Russia claimed fresh advances on the battlefield in east Ukraine.
The defence ministry said on Telegram that its troops had "liberated" the villages of Lozova in the northeastern Kharkiv region and Krasnoye -- called Sontsivka in Ukraine.
The latter is close to the resource hub of Kurakhove, which Russia has almost encircled and would be a key prize in Moscow's attempt to capture the entire Donetsk region.
Russia has accelerated its advance across eastern Ukraine in recent months, looking to secure as much territory as possible before US President-elect Donald Trump comes to power in January.
The Republican has promised to bring a swift end to the nearly three-year-long conflict, without proposing any concrete terms for a ceasefire or peace deal.
Moscow's army claims to have seized more than 190 Ukrainian settlements this year, with Kyiv struggling to hold the line in the face of manpower and ammunition shortages.
