Russian-controlled forces in eastern Ukraine are increasing their combat readiness and have carried out large-scale exercises, the intelligence directorate of Ukraine's defence ministry said on Tuesday.
Kyiv 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.
Russian-backed forces in the Donbass region of eastern Ukraine have fought Ukrainian troops in a conflict that Kyiv says has killed 14,000 people since 2014.
Russia is "increasing the combat readiness of the Russian occupation forces in the temporarily occupied territory in Donetsk and Luhansk regions," the defence ministry said in a statement, referring to Donbass.
The exercises that began on Monday include mobilised combat reservists, it said.
Russia's foreign intelligence agency on Monday compared current tensions with the build-up to a brief 2008 war in which Russian forces crushed those of neighbouring Georgia.
U.S. officials told Reuters it remained unclear whether Russian President Vladimir Putin has decided what action to take.
None of the U.S. officials said they expected an imminent attack, but they 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 Kremlin has said previously that U.S. media outlets that have suggested Russia is poised to attack Ukraine are being used in a disinformation campaign.
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.
(Reporting by Natalia Zinets; writing Matthias Williams, editng by Timothy Heritage)
