The Spanish government is investigating claims that Venezuela has granted citizenship to four suspected members of the Basque separatist group Eta.
The claims come at a sensitive time for the Basque peace process
Venezuelan nationality would help them evade extradition to Spain for trial.
Spanish Interior Minister Alfredo Perez Rubalcaba said the move, if confirmed, would be unacceptable.
Eta is blamed for the deaths of more than 800 people since it began an armed struggle for Basque independence in the 1960s.
The Vasco Press news agency reported that Eugenio Barrutiabengoa, Lorenzo Ayestaran, Miguel Angel Aldana and Jesus Ricardo Urteaga - who have lived in Venezuela for two decades - are to be granted citizenship.
Media reports in Spain say that Venezuela also struck a deal with Eta which allegedly involved paying thousands of euros in compensation to two other Basque activists over failures in an extradition process against them in 2002.
The BBC's Danny Wood in Madrid says that until now, diplomatic relations between Madrid and Caracas have been good, but these reports come at a sensitive time in the Basque peace process, launched by the Spanish government in June.
Street violence in the Basque region has increased in recent weeks, and French police have arrested Eta suspects in connection with a robbery at an arms factory, our correspondent says.