Anonymous warning precedes Spain bomb blast

MADRID, Spain (CNN) -- A bomb exploded in the northern port city of Zarautz in Spain's Basque region Sunday, slightly wounding three people.

Two police officers suffered minor ear damage and another person was hurt by shattered glass in the explosion, Spain's EFE news agency said.

Shortly before the blast, someone called transportation officials and said a bomb would go off near a construction company, EFE said.

Police cordoned off the area and asked residents to remain in their homes. Damage to the construction company building was not significant, EFE said.

The news agency did not say whether anyone claimed responsibility for the attack.

On Friday, the Basque separatist group ETA warned that its violent 40-year-fight for Basque independence "will continue" if the Spanish government doesn't offer "an adequate response to the root of the problem."

In the statement, ETA also claimed responsibility for four recent attacks, including the car bomb explosion at a Civil Guard barracks in northern Spain earlier in May that killed a Civil Guard member and wounded four others.

The warning came in an ETA statement published in the Basque newspaper Gara, just 10 days after police arrested the group's suspected leader in France. Five other suspected ETA operatives were arrested in France and Spain as well.

ETA is blamed for more than 800 deaths and thousands of injuries during its violent campaign since 1968. The European Union and the United States list ETA as a terrorist group.

All About Spain • ETA Separatist Group