Germany suspects that Russia is behind a series of unauthorized drone flights over German military bases, the country's Military Counter-intelligence Service (MAD) has said.

Some of the suspected spy activity is focused on the Bundeswehr's training of soldiers from Ukraine, MAD was cited by DPA news agency as saying.

"The number of such incidents has increased in the recent past," a spokesman for the Bundeswehr's Territorial Command told DPA.

He said there is increased vigilance among soldiers over possible spying attempts.

The spokesman said the conspicuous flights were observed at the beginning and end of training sessions for Ukrainian soldiers.

Suspicious activity has been reported in Wildflecken in Bavaria, Grafenwoehr and Idar-Oberstein in Rhineland-Palatinate, as well as at
the Altengrabow military training area in Saxony-Anhalt. 

No arrests have so far been made, which DPA said could be due to jurisdiction issues between the army and the police.

Bundeswehr members have jurisdiction over army properties, but any suspects apprehended would then be handed over to the police along with any drones seized, according to the Territorial Command.

Outside of Bundeswehr properties, however, the police are responsible for pursuing and arresting individuals.