Russian-controlled forces in eastern Ukraine are increasing combat readiness and have carried out large-scale exercises, the intelligence directorate of Ukraine's defence ministry said on Tuesday, Kyiv's latest warning of a potential new conflict.

Ukraine and NATO countries have expressed concern that Russian troop movements near Ukraine's borders could pave the way for an assault. Russia says it has no such plans and accuses Ukraine and the United States of stoking tensions.

Russia is "increasing the combat readiness of the Russian occupation forces in the temporarily occupied territory in Donetsk and Luhansk regions," Ukraine's defence ministry said in a statement, referring to two parts of Ukraine's eastern Donbass region held by Russian-backed separatists since 2014.

The exercises that began on Monday include mobilised combat reservists, it said.

Major fighting in eastern Ukraine ended with a ceasefire in 2015, but a conflict which has killed thousands of people remains unresolved and clashes take place regularly.

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.

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 Russian movements point towards a building crisis.

Ukraine has long sought Western military protection by applying for membership in NATO. Foreign Minister Dmytro Kuleba said on Twitter that Kyiv has had to defend its right to independence, and that NATO membership would "facilitate the development of the common security system".

Two refitted former U.S. Coast Guard patrol boats intended to bolster the Ukrainian navy arrived in Ukraine on Tuesday as a part of $2.5 billion package of assistance to Kyiv since 2014.