Kyiv on Friday accused Russia of increasing supplies of weapons, ammunition and military equipment to separatist-held territory in eastern Ukraine, ahead of key talks between Moscow and Washington.
The breakaway regions bordering Russia, collectively known as Donbass, have been locked in armed conflict with Kyiv's army since a Kremlin-backed insurgency following Russia's annexation of Crimea in 2014.
Ukraine's military intelligence service said that Russia "continues to increase the combat capabilities" of pro-Moscow separatists in the breakaway regions.
The Russian military this month has "secretly transferred by rail and road more than 7,000 tons of fuel, several tanks and self-propelled artillery units," to the regions, it said in a statement.
It added that Russia was also undertaking "active recruitment of mercenaries" who are being sent to separatist-controlled regions.
With tens of thousands of Russian troops massed on the Ukrainian border, fears are mounting that a major conflict could break out in Europe.
Moscow insists it has no plans to invade Ukraine but has at the same time laid down a series of security demands — including a ban on Ukraine joining NATO — in exchange for de-escalation.
A senior Russian lawmaker said Friday that the parliament will consider asking President Vladimir Putin to officially recognize the separatist statelets.
The conflict with separatists in eastern Ukraine has so far left more than 13,000 dead.
Kyiv and its Western allies accuse Russia of supporting the rebels by soldiers and arms — claims which Moscow denies.