Spain and the Netherlands have taken up air surveillance in Bulgaria amid soaring regional tensions, as NATO and Western allies fear Russia may be preparing to invade Ukraine.
The Spanish and Dutch will send up to nine fighter jets as part of an air policing mission in Bulgaria, a member of NATO. Bulgaria borders the Black Sea, as does Ukraine.
Bulgaria does not have any of its own fighter jets, Defence Minister Stefan Yanev said on Saturday.
The Bulgarian government is set to decide on the deployment of Spain's Eurofighter Typhoon jets on Monday.
There have been repeated Russian provocations on the Black Sea in recent years and tensions are high across the region.
The build-up of Russian troops along Ukraine's borders has triggered fears of an invasion. Extensive diplomatic efforts in recent weeks have failed to lead to a breakthrough.
Moscow has also demanded that NATO provide "security guarantees" that the defence alliance will not admit Ukraine or expand further east.
'Unacceptable' demands
On Friday, Bulgarian President Rumen Radev rejected Russian demands that NATO withdraw its troops from eastern member states such as Bulgaria, calling them "unacceptable and groundless."
Radev, who was re-elected in November, began his second five-year term on Saturday with a ceremonial military parade in central Sofia.
He was formerly an air force chief and fighter jet pilot.
