France and Spain were celebrating on Tuesday after the arrests of six alleged Basque separatists, including a man accused of seizing the reins of the militant group ETA after last month's capture of its military chief.

Police seized Aitzol Iriondo Yarza, known under the nom de guerre "Gurbitz", on Monday in the village of Gerde in southwestern France along with two suspected members of his "Commando Biscayne" militant cell, officials said.

Shortly afterwards, three more suspects were picked up at a highway toll booth near Irun, in northern Spain, following a joint operation hailed by both countries as the latest major blow against the violent group.

ETA is fighting for an independent Basque homeland spanning the Pyrenees in northern Spain and southwest France. Its 40-year campaign of bombings and assassinations has left more than 820 people dead.

Last month, France arrested Miguel de Garikoitz Aspiazu Rubina, known as "Txeroki" or Cherokee, and accused him of leading ETA military operations.

On Monday, France's interior minister, Michele Alliot-Marie, accused Gurbitz -- whom she said also operated under the alias "Balak" -- of taking command of ETA after Txeroki's capture left the group rudderless.

"The minister expressed her deep satisfaction, and that of Spanish Interior Minister Alfredo Perez Rubalcaba with whom she has just spoken, at these new arrests," said a statement from the French ministry.

Rubalcaba himself went further, linking Gurbitz to the murder last year of two Spanish undercover officers hunting ETA cadres in southern France and of last week's slaying of a Basque businessman.

Ignacio Uria Mendizabal, 71, was shot dead in Spain last Wednesday. His firm, Altuna y Uria, was involved in the construction of a high-speed rail network in the Basque region, a project opposed by ETA.

"It is not too bold to say that either Txeroki or Aitzol Iriondo was behind the order" to kill the businessman, Rubalcaba said.

"I don't know if any terrorist is thinking of taking over from Aitzol Iriondo. I can assure them that, from this moment, we are looking for you."

When Txeroki was arrested last month officials initially accused him of having killed the two undercover officers in Capbreton in December 2007, but he has only been charged with helping prepare the attacks.

Iriondo is the third top leader of ETA to be arrested this year, and French police boast of a total of 36 arrests of group members and sympathisers.

Aside from Txeroki, Javier Lopez Pena, the group's presumed overall leader, was arrested in the French city of Bordeaux in May in an operation that Spanish officials at the time boasted had "decapitated" the organisation.

France and Spain have stepped up cooperation to crack down on ETA since a special accord was signed in January allowing Spanish agents to operate in southwestern France.

Spanish police believe Txeroki, currently being held in a Paris jail, is connected to all the major ETA operations carried out since late 2003 when he took over the outfit's military operations.

That includes the Madrid airport bomb attack in December 2006 that killed two Ecuadorian men and led the Socialist government of Prime Minister Jose Luis Rodriguez Zapatero to end its tentative peace talks with ETA.