France has requested the extradition of an Argentine former officer, Alfredo Astiz, over alleged human rights crimes against French nationals during Argentina's military rule.
Argentine Captain Alfredo Astiz was dubbed the 'blond angel of death'
Mr Astiz - known as the "blond angel of death" for his alleged role in Argentina's "Dirty War" - was arrested on Friday.
France passed a life sentence in absentia on the 50-year-old former Argentine navy captain for his part in the murder of two French nuns in 1977.
Mr Astiz's detention came after President Nestor Kirchner signed an order allowing officers to be tried abroad - annulling a previous decree banning such extraditions.
A Spanish judge, Baltasar Garzon, has also requested the extradition of a total of 45 former military officers accused of crimes against Spanish nationals.
His list includes 1976 coup leader Jorge Videla and Emilio Massera, the head of the Navy School of Mechanics, which was known as a torture centre, as well as Mr Astiz.
Argentine authorities have now arrested 42 former military officers suspected of torture and murder during military rule between 1976 and 1983.
Observers say the officers may contest any extradition proceedings under amnesty laws passed in Argentina in the 1980s.
The BBC's Daniel Schweimler says that although Argentina's military rulers left office two decades ago, their seven years in office have cast a long shadow.
Official figures say 9,000 people were kidnapped, tortured and killed in what became known as the Dirty War - but most believe the real number to be closer to 30,000.
Argentine human rights groups have long demanded justice for those who tortured and murdered their left-wing opponents.
Many officers were initially prosecuted - then granted amnesties by the weak and frightened civilian governments that followed the military, our correspondent says.
Some of the wanted officers have been detained. Others were already under arrest in relation to human rights charges being investigated by Argentine judges.
One of the men on Judge Garzon's list, former member of the Naval Prefecture Juan Antonio Azic, is in a Buenos Aires hospital after trying to kill himself, according to the Spanish news agency Efe.
The decree annulled by Mr Kirchner had been introduced in 2001 by former President Fernando de la Rua.
Mr Kirchner came to office in May vowing to end impunity and to purge the military of those responsible for abuses.
Judge Garzon has led the way in trying to bring those suspected of human rights violations in Latin America to justice.
He spearheaded the unsuccessful campaign to extradite former Chilean dictator General Augusto Pinochet from London to Spain.
More recently he secured the extradition of former Argentine military officer, Ricardo Miguel Cavallo, from Mexico to Spain to face charges of genocide and terrorism.
No date has been set for his trial but it is expected to take place in 2004.