A Basque separatist militant arrested this week in France was the overall leader of the armed group ETA and a retaliatory attack is likely, Spain's interior minister said Wednesday.

Until now Spanish authorities have said Mikel de Garikoitz Aspiazu, whose alias is Txeroki, was in charge of ETA's commando units, which carry out attacks.

But Interior Minister Alfredo Perez Rubalcaba said in an interview with Cadena Ser radio that Aspiazu was actually ETA's top figure, in charge of overall strategy as well as ordering attacks.

He rose to the top position after the arrest of Francisco Javier Lopez Pena, then believed to be ETA's boss, in May near Bordeaux, France, the minister said.

"Txeroki ended up in charge of everything — the political apparatus, the so-called military apparatus. The one who ordered killings was Txeroki," Perez Rubalcaba said.

Police arrested Aspiazu, 35, on Monday in the French town of Cauterets, near the border with Spain, along with Leire Lopez, a woman who is also a suspected ETA member.

Spanish and French officials say Aspiazu is a top suspect in the shooting of two Spanish civil guards in December in the French resort town of Capbreton.

Following his arrest, Spanish security forces are on maximum alert because ETA will probably try to attack and show it is not in disarray, the minister said.

"We have to be prepared. One of these days ETA will try to show it is not weak, that even though Txeroki has been arrested, it can still act," Perez Rubalcaba said.

ETA has killed more than 800 people since the late 1960s in its battle to create and independent Basque homeland in northern Spain and southwest France.

It declared what it called a permanent cease-fire in March 2006 and began peace talks with the government of Prime Minister Jose Luis Rodriguez Zapatero. But the negotiations went nowhere and ETA detonated a car bomb at Madrid's Barajas airport in December 2006, killing two people.

Aspiazu and Lopez Pena are believed to have been opposed to the truce from the outset, and Aspiazu is described as having given the order to end it with the airport bombing.