About 70,000 people currently work in the country's defence industry, which specialises in ammunition for Soviet-era weapons and has seen its exports triple since Russia invaded Ukraine.

A fire broke out at a privately-owned ammunition depot near the southeastern town of Karnobat early Sunday, causing several explosions, but no casualties.

"There are additional levels of security that have been implemented for each of the defence industry sites, both state-owned and private," Economy and Industry Minister Bogdan Bogdanov told journalists on Monday.

"It's a key industry for Bulgaria," he said after visiting the largest state-owned VMZ Sopot factory in central Bulgaria.

The incident comes as NATO member Bulgaria is preparing to approve sending military aid directly to Ukraine, reversing its former policy of delivering arms to Kyiv via third countries.

The destroyed depot belongs to arms dealer Emilian Gebrev, whose company Emco was a target of similar incidents in the 2010s as well as in July 2022.

In a television interview Gebrev ruled out negligence as a possible cause of the fire, instead suspecting sabotage or trespassing.

Prime Minister Nikolay Denkov on Monday voiced concerns over the "explosions that were occurring periodically, especially over the past ten years".

"There was information that foreign intelligence services might have been involved," he said, urging prosecutors to present the results of previous investigations.

Bulgaria's prosecution announced in 2021 that it had established links between six Russian nationals in Bulgaria and four explosions at its arms depots between 2011 and 2020 allegedly aimed at preventing deliveries to Ukraine and Georgia.

In April 2015 Gebrev, his son and an executive from his company were victims of a poisoning attempt.

Three Russians were charged in 2020 in Bulgaria in absentia in connection with the attempted poisoning of Gebrev, one of them a Russian intelligence officer.