France has been helping Spain to catch Eta suspects
The man suspected of being the new military chief of the Basque separatist group Eta has been arrested in France, police say.
Aitzol Irionda, 31, was arrested along with two others near Gerde, south-west France, carrying guns and fake identity papers, French police said.
Spanish Interior Minister Alfredo Perez Rubalcaba described Mr Irionda as the "number one" Eta member.
The arrests follow last week's murder of a businessman in the Basque region.
It was the first attack linked to Eta since the arrest of its former military head, Mikel Garikoitz Aspiazu Rubina, also known as "Txeroki", last month.
"Aitzol Iriondo is presumably Txeroki's substitute," Mr Rubalcaba told a news conference in Madrid.
"As such we're talking about the military chief and number one member of the terrorist group Eta."
Mr Rubalcaba said the authorities would continue to pursue and arrest Eta members "until this finishes".
He told anyone considering taking over Mr Iriondo's role as head of the organisation: "I can assure them that, from this moment, we are looking for you".
Mr Iriondo is expected to be questioned by the police about the murder last December of two Spanish police officers, who had been working undercover in France.
The Spanish authorities will portray these arrests as further evidence that Eta is steadily being weakened, the BBC's Steve Kingstone in Madrid says.
But the group still has the potential to kill, and has pledged to continue its campaign of violence with a view to securing an independent Basque state, he adds.
Once a safe haven for Basque militants, France began arresting Eta suspects after the group called off a 15-month-old ceasefire in June 2007.
At least 36 people who were either Eta members or linked to the group have been arrested in France since the beginning of the year, according to interior ministry figures.
France and Spain have stepped up co-operation to crack down on the group since signing a special accord in January 2008 allowing Spanish agents to operate in south-western France.
The French Interior Minster, Michele Alliot-Marie, said the three latest arrests were proof of France's "resolute commitment in the fight against Basque terrorism".
Eta began its violent campaign for the independence of traditional Basque territories in northern Spain and south-west France in the late years of the dictatorship of Francisco Franco in the 1960s.