Russia said Monday that drone attacks in central Moscow and annexed Crimea could warrant a harsh response, after Ukraine claimed an attack on the capital.
"We regard what happened as yet another use of terrorist methods and intimidation of the civilian population by the military and political leadership of Ukraine," Russia's Foreign Ministry said.
"The Russian Federation reserves the right to take tough retaliatory measures," it added.
The ministry said the "West's focus on further aggravating the situation" in Ukraine was behind Kyiv's "brazen actions."
Russian officials said earlier that Ukrainian drones hit two buildings in Moscow and an ammunition depot in Crimea, the peninsula annexed by Russia from Ukraine in 2014.
A Ukrainian defense source told AFP the attack on Moscow was a "special operation" carried out by Kyiv's military intelligence.
The attack on Moscow, some 500 kilometers (310 miles) from Ukraine, comes after Kyiv vowed to retaliate for a Russian missile strike on the city of Odesa a day earlier.
The attack killed two people and damaged a historic cathedral in the UNESCO-protected city on the Black Sea.