Madrid - A bomb attack by suspected Basque terrorists in Spain has caused serious damage, the Basque Interior Ministry reported Saturday.

The attack took place overnight when a 5-kilo bomb hidden in a rucksack exploded in front of a courthouse in the town of Tolosa to the south of San Sebastian in the Basque region of northern Spain.

No people were injured in the blast which blew a 3-metre hole out of the facade of the courthouse and shattered all the windows in the five-storey building.

Several other buildings and parked cars were damaged in the attack.

Some 20 minutes before the blast, an anonymous caller gave a warning on behalf of the militant Basque separatist organization ETA.

It is believed that two people dropped off the rucksack outside the courthouse one hour before it exploded.

Police cordoned off the area following the warning.

Two weeks ago, one person was killed and 17 wounded in a series of ETA attacks in northern Spain.

In the space of 24 hours, ETA set off three large car bombs outside a military academy and residence, a police station and a bank.

ETA is listed as a terrorist organization by the UN, EU and US. (dpa)