Josu Jon Imaz, the chief executive of Spanish oil company Repsol, on April 27 asked European Union authorities to put an end to imports of Russian diesel to the bloc through third destinations.
"Diesel from Russia continues to be on European and Spanish markets," he said, adding that it was brought into Europe through third countries which made its origin unclear.
The European Union has banned Russian crude imports and oil products as part of its sanctions against Russia following the invasion of Ukraine.
Imaz called on the EU to be firm and enforce current sanctions.
"We ask European authorities to stop these imports through intermediaries," he said.