Late on Thursday Bulgarian media had reported the arrest of "persons suspected of spying for Russia".

One of those arrested had attempted to flee, according to television reports.

Russo-Bulgarian relations have been hit by several spy scandals in recent years.

Between October 2019 and the end of 2020, five Russian diplomats and a technical assistant at the Russian embassy were expelled from Bulgaria.

Investigators said they had cooperated with Bulgarians to get access to sensitive information.

The rows have soured relations between the two former allies, which had maintained their close cultural, historical and economic ties even after the fall of the communist regime in 1989.