A bomb exploded near a highway between the Spanish cities of Bilbao and Santander today following a telephone warning in the name of Basque separatist group Eta.
Police said the call warned of four bombs planted on roads but said there were no immediate reports of injuries.
The Basque road assistance organisation received a call just before 6 am Irish time from someone claiming to represent Eta. The bombs were said to be on the roads from Vitoria to Burgos, Pamplona to Logrono, Bilbao to Santander and Pamplona to Zaragoza.
The bombs came on the same day as outlawed separatist party Batasuna, associated with Eta, called a general strike in the Basque region to protest against the local government's refusal to allow tributes to be paid to two Eta prisoners who died in jail.
Eta, classed as a terrorist organisation by the European Union and the United States, has stepped up its bombings in recent weeks, quashing speculation fostered by Spain's Socialist government that it was close to declaring a truce.
