An explosion has reportedly occurred overnight in Spain's Basque region right next to a police station in Zarautz, a seaside resort near San Sebastian, police stated on Tuesday.

Preliminary investigations show that the explosion which caused a lot of damage but no victims might be linked to the armed Basque separatist group ETA.

The incident, which took place without an advance warning, hasn’t been claim yet by any terrorist group.

Police reports show that the explosion damaged a garage and cloakroom of the police station and smashed the windows of surrounding buildings. Police investigators are not sure yet what kind of a device was used for the detonation.

As the Basque police force became one of ETA's top targets – according to the Basque police trade union Esan – 13 suspects of ETA-linked street violence were arrested on Monday.

Since the 14-month ceasefire ended in June, ETA is suspected of other four attacks which occurred in the Zarautz region. The most brutal attack took places in August, when a car bomb was detonated on a police barracks in Durango in the Basque region. The explosion led to the injuring of two police officers.

Other cases of street violence were reported Tuesday throughout Spain as ETA supporters planted an explosive device at a car dealer in the Basque capital Vitoria and attacked a bank teller with petrol bombs in Arbizu, near Navarre region.

ETA - Euskadi Ta Askatasuna (Basque for "Basque Homeland and Freedom") – is held responsible for more than 800 deaths over the last four decades. As their group name suggests, they fight for an independent Basque country covering parts of what is now northern Spain and southern France.

ETA is past of a faction informally known as the Basque National Liberation Movement. This movement contains several distinct organizations that promote a type of leftist Basque nationalism. Other groups considered to belong to this movement are: the political party Batasuna, the nationalist youth organization Segi, the labor union Langile Abertzaleen Batzordeak (LAB), and Askatasuna among others. There are strong interconnections between these groups as double or even triple membership is common according to the Spanish Authorities.