Putin issued the order to start the previously unannounced maneuvers at 4 a.m. Moscow time (12.00 a.m. EDT) as he flew back from an international summit in South Africa, his spokesman, Dmitry Peskov, told reporters by telephone.

"These are large-scale unannounced test exercises," Peskov said, adding that 36 warships and an unspecified number of warplanes would take part. "The main goal is to check the readiness and cohesion of the various units."

He did not say how long the exercises would last.

Putin has stressed the importance of a strong and agile military since he returned to the presidency last May after four years as prime minister. In 13 years in power, he has often cited external threats when talking of the need for unity in Russia.

Russia's Black Sea fleet, whose main base is in the Ukrainian port of Sevastopol, was instrumental in a war with Georgia in 2008 over the Russian-backed breakaway Georgian regions of South Ossetia and Abkhazia.

Disputes with Kiev over Moscow's continued lease of the Black Sea navy base have been a thorn in relations with its former Soviet neighbor.

Peskov said that Russia is under no obligation to warn neighbors ahead of time of plans to hold the air and sea military exercises as long as fewer than 7,000 servicemen participated in the maneuvers.

(Reporting by Alexei Anishchuk; Writing by Alissa de Carbonnel; Editing by Douglas Busvine and Toby Chopra)