Russia is to withdraw its military officers from a joint Russia-Ukraine centre monitoring a fragile truce in the conflict-torn eastern Ukraine.
Russia's foreign ministry accused Ukraine of preventing the officers from carrying out their duties, saying they would leave on Tuesday.
Kiev said the move was a "provocation", seriously undermining deals to end the conflict with Russian-backed rebels.
It said three Ukrainian soldiers were killed by rebel shelling on Sunday.
Meanwhile, the separatists accused Ukraine's military of firing hundreds of shells on the rebel-held areas over the past 24 hours. They reported no casualties.
The UN says more than 10,000 people have died in Ukraine's eastern Donetsk and Luhansk regions since the conflict erupted in April 2014, soon after Russia annexed Ukraine's Crimea peninsula.
In a statement (in Russian), the Russian foreign ministry accused the Ukrainian side of the joint centre of "creating a tense moral and psychological" situation for the Russian officers.
It also cited restrictions on visiting the line of division and bans on contacts with the local population.
Ukraine's foreign ministry responded, saying in a statement (in Ukrainian): "We consider this unprecedented step by Russia as a new provocation which considerably undermines the Minsk agreements", referring to the Western-brokered deals to end the conflict.
Meanwhile, Ukraine's military warned that Russia's decision to pull out its officers could further escalate the conflict.
"The enemy may resort to aggressive offensive actions along the entire line of division," it said in a statement (in Ukrainian). citing information from the country's intelligence officers.
The joint Russia-Ukraine centre was set up in September 2014 to monitor compliance with the ceasefire regime.
Ukraine and Western nations accuse Russia of helping the rebels with regular troops and heavy weapons.
The Kremlin denies that, while admitting that Russian "volunteers" are helping the separatists.