The armed Basque group ETA set off a small bomb at a disused power station in northern Spain Tuesday, causing no injuries or damage.
The blast occurred at a power plant outside the northern city of Tarazona, El Mundo newspaper said on its Web site. The Interior Ministry confirmed they were investigating reports of a blast but had no other immediate details.
El Mundo said the blast had been preceded by a warning telephone call in ETA's name to authorities in the Basque region.
ETA has killed more than 800 people in a campaign for Basque independence since 1968. In recent months it has set off a series of small devices but its last lethal attack was in May 2003, when it killed two police officers.
Speculation has risen recently that the group may be prepared to call a cease-fire. But a statement from the group published Tuesday made no mention of an end to the violence, the AP reports.