A bomb exploded this morning at the offices of a far-right political party in Santona in the Cantabria region of northern Spain.
Police said a warning phone was received from Basque separatist group, 45 minutes Eta the device, which was left in a sports bag, went off at around 7 am Irish time.
The regional government confirmed there had been a blast at the offices of the Falange, a patriotic party founded in the 1930s which became the only official political organisation under the dictatorship of General Francisco Franco. The party is still active, and strongly opposes regional separatism.
Officials said there had been no injuries, but police had cordoned off the area and were assessing damage.
Eta has stepped up its bombings in recent weeks, quashing speculation fostered by Spain's Socialist government that it was close to declaring a truce.
The bomb coincides with reports that Basque companies have been subject to a new wave of extortion and that this time letters demanding money for Eta have contained photos of executives' families.
Interior Minister Jose Antonio Alonso is due to meet Basque employers later today to offer support, and possibly additional protection.
Eta has killed some 850 people in its almost four decade campaign for an independent Basque state. Its recent bomb attacks have been small devices, usually preceded by warning calls, and have caused no serious injuries.
Batasuna, the political party outlawed for its links with Eta , has called a general strike in the Basque Country and neighbouring Navarre tomorrow which will be a test of the extent of its popular support.