Russia has over 92,000 troops massed around Ukraine's borders and has been preparing for an attack by the end of January or beginning of February, according to media reports.
Ukraine has said Russian-controlled forces in eastern Ukraine have been increasing their activity and have carried out large-scale exercises in the occupied regions.
Russia is "increasing the combat readiness of the Russian occupation forces in the temporarily occupied territory in Donetsk and Luhansk regions," the intelligence directorate of Ukraine's Defence Ministry said in a statement on Tuesday.
The exercises that began on Monday include mobilised combat reservists, it said.
Kiev and NATO countries have expressed concern that Russian troop movements near Ukraine's borders could pave the way for a sharp military escalation.
Russia says it has no such plans and has accused Ukraine and the United States of stoking tensions.
US officials said the Russian movements point towards a building crisis.
They said Washington hopes to avert conflict by building international consensus about the threat, raising the possibility of potential sanctions should Moscow decide to act.
The head of Ukraine's military intelligence told the Military Times outlet this weekend that Russia had more than 92,000 troops massed around Ukraine's borders and was preparing for an attack by the end of January or beginning of February.