Madrid - Spain's Basque authorities have warned the regional police force that the militant Basque separatist group ETA may be planning an imminent large-scale attack against it, press reports said Thursday.

A part of ETA's Vizcaya complex, regarded as its most important structure, appears to have remained intact despite the arrests of 11 of its suspected members or collaborators, including its leader, last week, the Basque regional Interior Ministry wrote to the police force Ertzainza, instructing it to step up security.

ETA was thought to be planning an attack during some of the summer's regional festivities which draw large amounts of members of the public and police to protect them.

Arms caches discovered following last week's arrests have revealed that ETA possesses bomb detonators that are activated by their exposure to light.

ETA's recent attacks have included a bombing in the southern tourist resort of Torremolinos, which did not cause significant injuries.

ETA has killed more than 820 people in its four-decade campaign for a sovereign Basque state, four of them since it declared the end of a 14-month ceasefire in June 2007. dpa