KIEV - Ukraine detained Russia's military attache to Kiev on suspicion of spying and ordered the diplomat to leave the country, reports said on Thursday, as the former Soviet republic wrestles with a pro-Russian armed uprising in the east.

Interfax-Ukraine, citing the Ukrainian Foreign Ministry, said the unnamed diplomat was detained on Wednesday while undertaking "intelligence activities".

The ministry could not immediately be reached for comment.

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-organised 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 and has massed tens of thousands of troops on its western frontier with Ukraine.

Interfax said the diplomat had been ordered to leave the country.

"The military-naval attache 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 agency quoted the ministry as saying.

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 the worst civil turmoil in the country since independence in 1991, which have seen Moscow-backed president Viktor Yanukovich toppled by street protests, gunbattles in central Kiev and Russia's annexation of the Crimean peninsula.