Ukraine's defense ministry warned that Russia was stepping up its "combat readiness" in Ukraine.
In a statement on Tuesday, it said that Russia "is increasing the combat readiness of the Russian occupation forces in the temporarily occupied territory in the Donetsk and Luhansk regions."
It said that Russia ran "large-scale command and staff exercises" on Monday, including pulling up reserve troops.
It comes amid heightened warnings that Russia may be preparing to invade Ukraine.
The US is warning allies that Russia could decide to invade Ukraine in the next few months and could deploy around 100,000 troops to Ukraine, Bloomberg reported this week.
Experts told Insider's Ryan Pickrell that Russia invading Ukraine is a "very real possibility."
Russia annexed the Ukrainian peninsula of Crimea in 2014. Russian-backed separatists are also fighting in eastern Ukraine.
The head of Ukraine's defense intelligence agency told Military Times on Saturday that Russia already has around 100,000 troops on its border with Ukraine, and that Russia was preparing for a possible attack that could come in early 2022.
