France on Thursday vowed a "merciless" campaign to stamp out ETA on its soil after a member of the Basque separatist group was accused of shooting dead a French policeman.

"The mobilisation of the French police and gendarmes against this terrorist organisation will be total and merciless," Sarkozy said after meeting the family of the 52-year-old officer who was gunned down on Tuesday.

"Let no one imagine that the territory of the French republic is a quiet rear-base for the terrorists and assassins who kill, as ETA has shown it can for decades," he said in Damarie-les-Lys, scene of the shooting southeast of Paris.

French anti-terrorism police arrested a 27-year-old man who identified himself as an ETA member and were hunting five others, including a woman, after the murder, a judicial official said on Wednesday.

The official named the suspect as Joseba Fernandez Aspurz.

French investigators said they were working on the assumption that ETA was responsible for the policeman's killing but that there had not yet been a claim of responsibility from the Basque group itself.

The killing of the policeman, Jean Serge Nerin, was the first deadly attack on a French officer by militants of the separatist group.

Sarkozy spoke by phone with Spanish Prime Minister Jose Luis Rodriguez Zapatero on Wednesday and agreed to "redouble" efforts to root out ETA.

Banned as a terrorist group by the European Union and the United States, ETA is blamed for 828 deaths in its 41-year campaign for independence for the Basque region of northern Spain and southwestern France.

Around 100 alleged members of ETA were arrested in 2009 and some 30 more since the start of this year, many of them in France.

Sarkozy said Zapatero would attend Nerin's funeral on Tuesday.