Five Eta leaders have been arrested in France in the last two years.
Security forces believe Eta has killed more than 850 people in its campaign for independence for the Basque Country in northwestern Spain.
But in recent years the movement has been severely weakened by hundreds of arrests.
The increase in the number of arrestsis said to be due to improved co-ordination between the authorities in Spain and France, which Eta has long used as a hideout.
Eta, whose initials stand for Euskadi ta Askatasuna, or Basque Homeland and Freedom in the Basque language, is considered a "terrorist" group by Spain, the European Union and the US.
For four decades the group has waged a bloody campaign for independence for the seven regions in northern Spain and southwest France that Basque separatists claim as their own.