Judicial officials in El Salvador on Thursday formally notified four retired soldiers being detained that Spain was seeking their extradition over the murder of six Jesuit priests in 1989, one of their lawyers said.
The attorney, Lisandro Quintanilla, however argued that the request should not be granted as the matter had already been judged by Salvadoran courts in 1991.
"We are going to insist with a motion that they not be extradited, and we will exhaust all legal avenues necessary," Quintanilla said.
El Salvador's Supreme Court is to rule on the extradition request.
The four -- retired colonel Guillermo Benavides, ex-deputy sergeants Antonio Avalos and Tomas Zarpate, and ex-corporal Angel Perez Vasquez -- were arrested in February on the basis of an Interpol notice issued by Spain and are being held.
Another Salvadoran ex-soldier connected to the murders, colonel Inocente Orlando Montano, is being detained in the United States, where he gained residency by lying about his military record.
Another 12 Salvadoran soldiers sought by Spain over the killings are fugitives.
The six priests -- five of them Spanish-born -- were slain at their residence on a university campus in El Salvador's capital by uniformed armed men on November 16, 1989, during the country's civil war.
The priests' domestic worker and her 16-year-old daughter were also murdered, so there would be no witnesses.
A subsequent Truth Commission established that the assailants were members of an elite army unit trained by the US military. They were ordered to carry out the operation because the priests were thought to have helped left-wing rebels.
The soldiers used a captured rebel assault rifle and left a message in an effort to make it looks like the insurgents had carried out the crime.
A 1993 amnesty law in El Salvador swept away sentences handed down to some of the former soldiers by a Salvadoran court, notably scrapping a 30-year term Benavides had been serving.
But in 2009, a Spanish court opened a lawsuit over the murders.
In 2011, Spanish judges indicted 20 former Salvadoran soldiers on war crimes, including all those in detention, and demanded their arrest.
