A NATO member since 2004, the Balkan country is obliged to keep at least one squadron of jets ready for action, but the number of its ageing Soviet-built MiG-29s has progressively dwindled to just seven, prompting repeated alerts about drastic shortages in pilots' flying hours.

The defence ministry had already sought offers for new or used fighters in late 2016 and shortlisted Sweden's Gripen as its preferred option, but the procedure was sidelined by the new conservative cabinet, which renewed earlier this year a request for proposals from other suppliers.

In response, the US made two bids -- for new Boeing F-18s and F-16s by Lockheed Martin, while Sweden offered new Gripen fighters and Italy proposed second-hand Eurofighter jets, deputy defence minister Atanas Zapryanov said as the proposals were unveiled.

He did not provide details about the bids however, saying only that a defence ministry committee and another group of experts would review them before the government chose who to begin final talks with.

Although there was no deadline, Zapryanov said "our desire is to do it quickly. We have a budget that we want to spend."

Bulgarian lawmakers agreed in June to spend 1.8 billion leva (920 million euros, 1.0 million dollars) for at least eight new or used fighter jets, with experts commenting that amount would not be enough to buy F-18s.