Negotiations are taking place between Moscow and western countries to exchange the two suspected Russian spies detained in Slovenia for a person or people currently in jail in Russia, the Guardian reported from Ljubljana on 24 March, citing an unnamed source.

The couple suspected of spying for Russia were apprehended in Slovenia in early December. Carrying Argentinian passports, they had adopted false identities as Maria Rosa Mayer Munos and Ludwig Gisch and lived with their two young children in Črnuče, a quiet suburb of Ljubljana.

According to the report in the British newspaper, their acquaintances described the couple as ordinary and nice. Their neighbours said "the children could often be heard playing in the garden, shrieking in Spanish".

When the couple were arrested, their two children were taken into social care. Police also raided an office owned by the couple, finding an "enormous amount of cash; so much, in fact, that it took hours to count", the Guardian was told by a source with knowledge of the investigation.

Sources in Ljubljana also told the Guardian that the couple were in fact elite Russian spies known as 'illegals' and worked for Russia's SVR foreign intelligence service.

The paper explains that unlike 'legal' Russian intelligence officers, who are disguised as diplomats at Russian embassies across the world, the illegals operate without any visible links to Moscow.

"If the couple are indeed SVR illegals, it will be the first such case aired publicly since 2010, when the FBI rounded up a group of 10 in the US after tipoffs from a mole inside Russian intelligence," the newspaper writes.

Foreign Minister Tanja Fajon told reporters during a visit to New York on 23 March that the indictment against the two alleged Russian spies had become final and they had been remanded in custody.

The Guardian quotes one source with knowledge of behind-the-scenes manoeuvres as saying that in informal conversations after the arrests, Moscow had quickly accepted the couple were intelligence officers.

"Even as preparations are under way for a trial in Slovenia, backdoor negotiations are taking place between Moscow and western countries to exchange them for a person or people currently in jail in Russia," the Guardian quotes the source as saying.

The Foreign Ministry has summoned Russian Ambassador Timur Eyvazov over the matter, expressing its protest in the strongest terms to him on Thursday.

The source also told the British newspaper that the majority of the couple's activity was not in Slovenia, which as a country with a "weaker counterintelligence environment" was a perfect base for the couple to be able to travel through most of Europe without border checks.

In custody, the suspects have said little. "They have taken it stoically. It's obvious they are pros. But they are not talking," the Guardian quotes one source, who added that negotiations on an exchange were taking place at a high level.

"Now we will see how important these people really are to Russia. This is big game now; it's clear that Slovenia is just a proxy here," the source told the Guardian.