Police said they were checking all vehicles crossing into France and paying particular attention to heavy goods vehicles. The checks had led to queues on the A63 motorway at the border town of Biriatou, a police official said.
French Interior Minister Nicolas Sarkozy also sent a telegram to his Spanish counterpart Angel Acebes with an offer of police help.
French and Spanish police have cooperated closely during a recent crackdown on ETA Basque separatist guerrillas who want a separate homeland and consider this includes part of southwestern France.