In a statement, Ukraine's Foreign Ministry said the unnamed diplomat was detained on Wednesday while undertaking "intelligence activities". (Agencies)
Ukraine accuses Russia of orchestrating the fall of towns and cities across its industrial east to pro-Russian separatists over the past month, spearheaded by well-organized gunmen in masks and military fatigues.
Russia denies having any part in the rebellion, but has warned it reserves the right to intervene to protect ethnic Russians - following its annexation of Crimea in late March - and has massed tens of thousands of troops on its western frontier with Ukraine.
"The military-naval attaché of the embassy of the Russian Federation in Ukraine is declared persona non grata in connection with his actions, which are not in accordance with his diplomatic status," the ministry said.
The diplomat was ordered to leave, though the statement mentioned no deadline. There was no immediate response from Moscow, which like Kiev is observing the May 1 holiday.
Ukraine's pro-Western leaders conceded on Wednesday they were "helpless" to counter the fall of government buildings and police stations to the separatists in the Donbass coal and steel belt of eastern Ukraine, source of around a third of the country's industrial output.
Having seized key buildings in the capital of the easternmost province, Luhansk, on Tuesday, gunmen took control at dawn on Wednesday in the nearby towns of Horlivka and Alchevsk.
In Donetsk, the biggest city to fall, mainly Russian-speaking separatists have declared a "People's Republic of Donetsk" and called a referendum on secession for May 11, threatening to undercut a planned presidential election in Ukraine two weeks later.
Ukraine hopes the presidential poll will help restore order after five months of civil turmoil that saw Moscow-backed president Viktor Yanukovich toppled after street protests and gun battles in central Kiev, and Russia's subsequent annexation of the Crimean peninsula.
Pro-Western authorities that took power with Yanukovich's ouster accuse Russia of planning to disrupt the presidential election, create instability and frustrate the new government's hopes of Western integration.
Overnight, the state security guard, responsible for securing key government sites and officials, carried out a small drill in central Kiev. Four armored personnel carriers trundled through the streets to parliament, where several dozen troops took position as if responding to a threat.
The guard's commander, Valery Galetey, said they were training for possible "provocations" during the May 25 election.
On Tuesday, Oleksander Turchinov, Ukraine's acting president, said the armed forces were on full alert for a Russian invasion.
That prompted a return volley from Moscow, where the Foreign Ministry demanded that Kiev "immediately ceases the bellicose rhetoric, which is aimed at intimidating its own population".
In a statement, Ukraine's Foreign Ministry said the unnamed diplomat was detained on Wednesday while undertaking "intelligence activities".