Bulgaria’s deputy defense minister confirmed on Tuesday that some military pilots will not take part in planned training this week.

Some of the pilots from Graf Ignatievo air base will not perform training flights because of insecurity

said Deputy Defense Minister Atanas Zapryanov.

The pilots have blamed safety concerns related to aging Soviet-built MiG-29 aircraft. But Zapryanov insisted that the jets were safe and were being used to as part of NATO’s air-policing mission.

According to Zapryanov, the pilots had further cited poor preparedness due to lack of flying hours as the reason to boycott training.

Outdated fleet

Just seven of Bulgarian air force’s 16 MiG-29s aircraft are in good flying condition – a reason why the Balkan country launched a €770 million ($905 million) tender to buy eight new fighter jets.

But the plan to buy Sweden’s Gripen aircraft earlier this year was sidetracked by Prime Minister Boyko Borisov’s conservative government.

Borisov’s cabinet is now expected to launch a new tender. In the meantime, it plans to refurbish its fleet of 16 MiG-29s and some 20 Su-25 bombers. The Defense Ministry has received six used MiG-29 engines, as well as four new ones in a deal with Russia but their use have been delayed by documentation-related issues.

Bulgaria, a NATO member since 2004, is obligated to maintain at least one squadron of 12 jets in good flying condition. With more than half the jets in the existing fleet in bad condition, Bulgarian parliament has authorized NATO to help protect the country’s air space.