Bulgaria's defence ministry said on Friday it would repair six of its 19 Soviet-built MiG-29 fighter planes in Poland, in a move to reduce its dependence on Russia.

"Bulgaria and Poland express their strong political will for cooperation especially about the maintenance of the MiG-29 fighters," the defence ministry said in a statement after the signing of a letter of intent with Warsaw.

"The agreement with Poland envisages the repair of the engines and aggregates of three aircraft by mid-2016 and those of another three by end-2016," it added.

The repairs will be performed at the Polish state company WZL-2.

"Currently the Bulgarian Air Force has at its disposal four MiG-29 in good working order and with enough resource. After mid-2016 it could be left with just two," the statement said.

"In order to avert such a situation, Bulgaria must find a way to repair the engines of its MiG-29s," it added.

As a NATO member Bulgaria has an obligation to keep at least one squadron of 12 planes in good fighting order.

But it has constantly complained about delays in the implementation of a framework agreement with Russia's RSK MiG due to expire next month.

In a move to bypass the Russians, the defence ministry turned to Poland.

"This decision has a huge geopolitical importance," Polish Defence Minister Tomasz Siemoniak said after the signing ceremony.

In the meantime, RSK MiG's representative in Bulgaria Alexander Alexandrov argued in the Bulgarian press that the Polish company did not have a licence for the repairs.

Bulgaria's Defence Minister Nikolay Nenchev did not comment while the statement only said that Bulgaria has received guarantees from Poland about the quality of the repairs.

The details around the deal and its cost were due to be spelled out in another agreement that has to be approved by the parliaments of both countries.

Bulgaria has long planned to buy a minimum of eight new or used fighters to replace its ageing fleet of Soviet-built aircraft but a final decision was postponed for financial reasons.