Six Russian Sukhoi-27 fighter jets and three transport planes have been deployed at Bobruisk airfield in Belarus. Earlier this week Belarus President Aleksandr Lukashenko invited Russian forces to fend off potential NATO threat.

“In case of continuing build-up of military forces in countries bordering Belarus, the country will take adequate response measures,” says a statement from the country’s Defense Ministry in Minsk.

NATO is sending 12 F-16 craft in Poland, in the wake of the Crimea crisis, and Polish President Bronislaw Komorowski has asked for an even greater US presence. Baltic states have also requested military assistance from NATO.

Just like their NATO counterparts, the Russian aircraft in Belarus will participate in a joint training exercise.

“The joint air defense system [between Russia and Belarus] is intended to guarantee collective security in Eastern Europe,” the Belarusian Defense Ministry said.

Russian army's chief of staff Valery Gerasimov informed chairman of NATO's Military Committee, Knud Bartels, of the developments in a phone conversation on Thursday evening.

According to previous military agreements, Russian and Belarusian troops can move freely throughout both countries, though it is unusual for them to be stationed abroad for any prolonged period of time.

Aleksandr Lukashenko has expressed repeated concern about NATO’s eastward expansion since his ascension to the presidency of the 9-million-strong state in 1994, and has been one of Russia's staunchest allies.

All the same, following the emergency of a possibility of Crimea joining Russia following a referendum on March 16, the Belarus leader has spoken out in favor of “maintaining Ukraine’s territorial integrity”.