SOFIA (Reuters) - Bulgaria has put on high alert or deployed its air force about 30 times in two months in response to a recent spike in Russian military aircraft flying near its aerial borders on the Black Sea, its defense minister said on Tuesday.

Both the West and Russia have carried out a series of military drills as a show of force in the worsening standoff over Russia's annexation of Crimea, which has sparked retaliatory sanctions from Washington and Brussels.

NATO said it was "considering all options" as it studied new steps to bolster its military presence in eastern Europe on Tuesday, while saying it saw no sign that Russia was withdrawing tens of thousands of troops from the Ukranian boarder.

Despite its longstanding friendship with former Cold War ally Russia, Bulgaria joined NATO 10 years ago and has twice participated in navy drills with a U.S. warship in the Black Sea since the Ukraine crisis. It is also currently hosting a two-week military exercise that includes Ukrainian and U.S. forces.

"I can only guess what is the goal of these flights," said Defense Minister Angel Naidenov. "But when we have about 30 cases which promoted either takeoffs or bringing our jets on higher alert in the last couple of months, it is worth to be very vigilant," Angelov told reporters.

President Rosen Plevneliev, who is also commander in chief of the Bulgarian army, said Bulgaria's ageing Mig-29 jet fighters had been deployed 2-3 times a week in recent months, compared with a previous rate of 2-3 times a year.

Russia may be deliberately provoking such flights to exhaust the flying capacity of Bulgaria and other's Russian-made jets, Plevneliev said, adding that Romania, Turkey and Bulgaria should boost their cooperation in air policing.

"At the moment, one Russian air plane forces the take-off of two Romanian, two Bulgarian and one Turkish planes. This is quite ineffective," Plevneliev said.

Bulgaria has been considering buying new jet fighters and replacing its Soviet-era military fleet, but has delayed the process due to financial constraints.

(Reporting by Tsvetelia Tsolova; Editing by Matthias Williams and Catherine Evans)