A bomb has exploded outside a court in the town of Tolosa in Spain's northeastern Basque region after a warning call from the Basque separatist group Eta, public television TVE reported.

The bomb went off at 1.15am on Saturday (2315GMT), about half an hour after a man purporting to speak for Eta called the Basque traffic department to warn of an imminent blast, the television station said.

The bomb had been left in a rucksack on the steps of the court, the Europa Press news agency reported, citing local police who had cordoned off the area.

Eta, which is blamed for the deaths of over 820 people in its 40-year campaign for an independent Basque homeland, traditionally calls the traffic department to warn of attacks.

The group's last victim was a soldier who was killed when a car bomb exploded outside a military school in the northern autonomous Cantabria region on September 22.

The attack was one of three in northern Spain over that weekend blamed on Eta and in which 11 people were also wounded.

The attacks came as the Basque region planned to hold a referendum on self-determination, a vote which Spain's constitutional court has declared illegal.