Bulgaria's foreign ministry said Wednesday that it had ordered the expulsion of the diplomats, who Russia said worked in its trade mission.

The Russian foreign ministry in a statement expressed "regret" and said it viewed the decision as a "deliberate attempt to harm constructive Russia-Bulgaria cooperation.

It said it would announce retaliatory measures later.

The order for the pair to leave within 72 hours came after the Bulgarian public prosecutor alleged the Russians had been seeking out classified information on military modernisation plans since 2016.

EU member Bulgaria was once an ally of the USSR but ties have cooled recently and last year saw the ejection of three other Russian diplomats over allegations of obtaining classified information.

Three Russian nationals have been charged in absentia over the attempted poisoning in 2015 of a Bulgarian arms manufacturer, Emilian Gebrev, his son and another associate.

Russia has recently had a number of its diplomats expelled from several European countries, responding with tit-for-tat measures.

Norway expelled a diplomat in August after arresting a Norwegian man suspected of passing sensitive information to Moscow.

In the same month, Austria expelled a diplomat as media reported he had been involved in economic espionage for years.

Also in August, Slovakia expelled three diplomats for a "serious crime" with local media citing a possible link with the killing of a former Chechen rebel in a Berlin park last year.