Ukrainian officials detained a Russian military attache, which authorities suspect of spying.

The Ministry of Foreign Affairs in Kiev, though not initially disclosing the man's identity, said he was ordered to leave the country and was about to do this "in a short time".

Diplomatic representatives added he had been declared "persona non grata" for activities "not corresponding to his diplomatic status" in accordance with the Vienna Convention on Diplomatic Relations signed 1961, UNIAN news agency has reported.

Later Foreign Ministry reports suggested the alleged spy's name was Kirill Kolyuchkin, a Russian Navy attache, had been detained on April 30 while "pursuing agent activity". No immediate comments were made by Russian officials.

The news comes in weeks of heightened tensions between Kiev and Moscow over the unrest in Eastern Ukraine.

Ukraine's President Oleksandr Turchynov announced Wednesday the country's troops were on "full combat alert" following the developments in eastern Ukraine and the buildup of around 40 000 Russian troops near the Ukrainian border. He also admitted his forces could not tackle the unrest fueled by pro-Russian separatists who say their regions are to hold referendum on changing status within Ukraine or even breaking away to join Russia, as Crimea did on March 16.

In April, Moscow launched fresh military drills at its western borders with Ukraine, and representatives at the UN defended Russia's right to intervene in its neighbor if Kiev continues what the Kremlin calls a "violation" of the Russian-speaking population.

Over the last days the West, for its part, has adopted tougher sanctions against Russian officials by imposing travel bans and asset freezes.

Russia and the West accuse each other of flaring violence in the east and south of Ukraine, while separatists in the country's Kharkiv, Luhansk, and Donetsk regions continue occupying government buildings and maintaining roadblocks to prevent Kiev forces from entering towns and cities.