Security along the border between France and Spain has been tightened following this morning's attack on three Madrid train stations.
The French Basque region has long been a haven for militant Spanish Basques, although it has largely been spared the violence that has scarred the Spanish Basque provinces, just across the border.
Police stopped people on foot and searched vehicles, creating traffic jams at several checkpoints between the two countries, including Hendaye and Behobie in France.
President Jacques Chirac condemned the Madrid attacks and pledged "solidarity with Spain in fighting against this abominable scourge".
Cooperation between the two countries has been stepped up over the last few years. France has arrested numerous Spanish ETA members hiding out in the French Basque region, in the extreme southwest near the Pyrenees.
ETA is also known to have used France to supply its network with explosives. It allegedly joined up with the Breton Revolutionary Army, a tiny separatist movement in Brittany in western France, to steal eight tons of dynamite from a warehouse in 1999.
Some of the explosives are thought to have been used in attacks by ETA and the Breton group.
AP