"Due to the shortened legislative procedure, a series of important issues remained unclear such as costs, guarantees, delivery times, forfeits, indemnities," Radev said Tuesday.

Given the strategic importance of the agreement, this was not acceptable, he added.

Under the $1.3-billion (1.1-billion-euro) package deal with the US, Bulgaria's government agreed to buy eight F-16 Block 70 multi-role fighter jets for its airforce.

The deal also included missiles, training and support, making it the country's biggest military equipment purchase since the fall of communism three decades ago.

The defence minister had already signed the deal and the new aircraft were scheduled for delivery between mid-2023 and early 2024 to replace the NATO member's ageing Soviet-built MiG-29 fighters.

Parliament ratified the costly purchase in a "fast-track" procedure last Friday that allowed lawmakers to scrap discussions between the required two votes on the deal.

But the high price of the purchase triggered heated, if shorter, debates initiated by the socialist opposition, who wanted more details about the deal -- while not objecting to the need to buy more aircraft.

Radev cited these debates as a sign of the "lack of national consensus and conviction in the mutually beneficial terms of the agreement".

The president is a former pilot and air force commander and as president is also commander-in-chief of the army. He favoured choosing a cheaper option, such as Sweden's Gripen fighters.

He has also argued that other countries, such as Slovakia and Bahrain have paid less for the US fighter in recent deals.

On Tuesday, he argued that Bulgaria needed a multifunctional aircraft with a full package of arms, equipment and training, but added: "The public deserves a categorical answer if this is indeed achieved with the signed agreements."

Parliament will now have to hold another debate and vote again on the deal.

But lawmakers are expected to approve it and this time Radev will have to sign it into force.