The Foreign Ministry has summoned the Russian ambassador after the indictment against two alleged Russian spies who were apprehended in Slovenia in December last year became final and the pair were remanded in custody.

"The indictment against two Russian citizens is final today. This means detention has been extended," Foreign Minister Tanja Fajon told reporters on the sidelines of the UN Water Conference in New York on 23 March.

Fajon said the pair were charged with two criminal offences, of espionage and of certifying false content in official documents.

Based on the indictment becoming final, the Foreign Ministry summoned Russian Ambassador Timur Eyvazov for a discussion, which took place today. The ministry said it had voiced its protest in the strongest terms.

Espionage carries a prison sentence of up to five years and certification of false content in official documents up to three years in prison.

The two alleged spies, a man and a woman, were apprehended by Slovenian authorities in early December on a tip-off from SOVA, the national intelligence and security agency, which had identified them with the help of NATO and EU partners.

They lived in Slovenia under false identities as Ludwig Gisch and Maria Rosa Mayer Munos, a 38-year-old couple from Argentina. They have two children.

They ran a real-estate agency and an antiques business in Ljubljana as a cover-up, both of which companies were registered in 2017.

They reportedly used Ljubljana as a base for spying in Slovenia and other countries for at least six years.