Miguel de Garikoitz Aspiazu Rubina, also known as Txeroki, said ETA "regrets the damage" caused to victims as he appeared along with nine other suspected ETA members in a Paris court on charges of kidnapping a Spanish couple and their son in France in 2007.
A Spanish court in 2011 sentenced him in absentia to 377 years in prison for 20 attempted assassinations.
Txeroki, who was arrested in France in 2008, read out a statement in the Paris court calling on the French government to seize the opportunity to resolve the Basque conflict.
ETA is blamed for more than 800 deaths in a four-decade campaign of bombings and shootings for independence for the Basque Country of northern Spain and southwestern France.
Considered a terrorist group by the European Union and the United States, it announced in October 2011 that it was giving up its armed struggle. But it has yet to formally disarm, and the Spanish government has refused to hold talks with its leaders.