Aspiazu Rubina, 35, known by alias "Txeroki" or "Cherokee", was captured in Cauterets, a spa and ski resort in the Pyrenees in France, near Spain's autonomous Basque region. An unidentified woman was also detained.

Aspiazu Rubina has been the chief of ETA military units for several years. He is suspected of several killings, including shooting dead two Spanish officers in France in December last year, French and Spanish officials said.

French President Nicolas Sarkozy welcomed the arrest. He said "this arrest shows once again the strong commitment of the French police and the excellent collaboration between France and Spain in the fight against Basque terrorism."

"This is a big step forward," Spanish Prime Minister Jose Luis Rodriguez Zapatero said in a statement read on television Monday. "With this arrest ETA has suffered a hard blow to its organization and its capacity. Today, ETA is weaker and Spanish democracy is stronger."

ETA began its violence for independence of Basque Country in Spain since the 1960s. The violence has killed more than 800 people. Just six months ago before this arrest, the overall leader of ETA Francisco Javier Lopez Pena had been captured.