A small bomb exploded overnight at a highway building site in Spain's northern Basque region.

The explosion near the town of Orio caused no injuries but damaged four vehicles and six shacks used by work crews.

The damage was discovered this morning when workers arrived at the site.

The authorities are blaming the Basque separatist movement ETA for the explosion.

Anti-terrorism judge Baltasar Garzon yesterday ordered the detention of seven people on charges of belonging to an ETA cell suspected of carrying out the majority of the group's attacks since it called off its ceasefire in June 2007.

All seven, including the suspected leader of the cell, Arkaitz Goikoetxea, had been arrested in police raids last week.

Judge Garzon released three others who were held as part of the same operation, two without conditions and one on bail.

ETA set off four bombs at two northern seaside resorts, Laredo and Noja, on 20 July, slightly injuring a woman who was struck by a rock sent flying by one of the blasts.