Caches included 3 tonnes of explosives and thousands of rounds of ammunition says group which mediated between ETA and French authorities.
Group gives French authorities information on location of arms caches as part of promise to give up all remaining arms by Saturday April 8th.
Activists say arrest of five people in French Basque Country was a political move to prevent the destruction of part of the organisation's arsenal.
Mikel Irastorza, aged 41, who has been on the run since 2008, was detained in a house in Ascain, a village near the Pyrénées mountains.
Two of six members of ETA are accused of officer's murder, the others with theft and kidnapping, after a gun battle with Paris police in 2010.
French officials said Iratxe Sorzabal had been wanted for 'at least 10 years' while David Pla had been sought by police since 2011.
One of the men, arrested near the south-west town of Bayonne, is wanted in connection with the shooting in 2010 of a French policeman.
Interior minister Manuel Valls said five of the six suspected members of the Basque separatist group arrested were on the 'most wanted' list.
Woman arrested with her boyfriend in Macon is described as leader of the ETA Basque terrorist movement's killer squads.
The military leader of the Basque separatist group ETA has been arrested in France along with an alleged accomplice, Spanish officials say.