The Spanish government said Friday it had opened a probe into the possible importation of Russian diesel via third countries to avoid sanctions imposed over Moscow's invasion of Ukraine.

The European Union banned Russian crude imports from December 5 and Russian oil products from February 5 in an attempt to deprive Moscow of energy revenue.

But experts and oil firm executives suspect some Russian diesel is still arriving in EU member states after first arriving through other nations to mask its origins.

Spanish daily newspaper El Mundo reported Friday that at least two ships are suspected of having brought Russian fuel to Spanish ports from Morocco this year.

The report came a day after the chief executive of Spanish energy giant Repsol, Josu Jon Imaz, said Russian diesel was still coming into EU states, including Spain, via third nations.

Spanish Energy Minister Teresa Ribera said an initial inquiry showed all documentation was in order for the shipments and the fuel was not Russian, but the government would continue checking.

"Faced with the least suspicion, it is necessary to verify if the imported products come from the indicated place or from another country, if there is any irregularity," she added.

Ribera will write to the European Commission to propose the creation of a new certification and traceability system for oil imports, requiring evidence of the ports and refineries that supplied the fuel, her ministry said in a statement.

In Rabat, three parliamentary opposition groups recently called for a probe over "doubts and suspicions" about the import of Russian diesel by fuel companies and its "possible resale... to countries that prohibit its import".

Their bid to set up an investigative commission foundered after failing to secure a majority vote.

Last month Moroccan government spokesman Mustapha Baitas said the share of Russian diesel in Morocco's imports stood at nine percent, as it had done "for several years".