A car bomb exploded in an industrial park in central Spain last night following a warning in the name of armed Basque separatist group ETA, local media reports said.
Officials could not immediately be reached for comment, but Spanish news agency Europa Press said there were no injuries.
The explosion followed a warning call in the name of ETA to the Basque highway agency - a method often used by the outlawed group to warn of bombs. If confirmed it would be the first ETA bombing since late July and would dash growing peace hopes in Spain.
ETA did not stage its usual August bombing campaign at holiday resorts this year, leading to newspaper reports that a truce could be imminent.
The explosion was near the small town of Berrocalejo de Aragona, a few miles east of the historic walled city of Avila, which lies to the west of Madrid.
Media reports said there was a police academy in the area, which was sealed off by police after the explosion.
ETA has killed nearly 850 people since 1968 in a bombing and shooting campaign for an independent Basque state in northern Spain and southwestern France. Spain, the United States and the European Union consider it a terrorist organisation.
The group has been weakened in recent years by a sustained police crackdown that has led to hundreds of arrests in France and Spain.