Authorities in Spain's Basque Country fear separatist guerrilla group ETA will attempt a big attack soon and have ordered police to increase security measures, police said on Wednesday.

The rare warning, contained in an internal police memorandum later published on the police website, follows a spate of small bombings blamed on ETA at seaside resorts that are timed to hurt Spain's tourism sector during its high season.

They said Wednesday's warning was based on information gleaned after the arrest of 10 suspected members of ETA's "Vizcaya cell" on July 22.

Police said on Saturday ETA were planning an attack against police at a fair in the Basque town of Getxo set for Thursday, and they feared the group would now attempt a similar attack.

"The Interior Ministry warns that the terrorists could try and attack in a similar way: somewhere holding a fair with a massive public presence and therefore a greater police presence."

A small bomb exploded in the southern resort of Torremolinos on Tuesday and the government blamed ETA for what it said was their fifth such attack on holiday resorts in 10 days.

The group were also blamed for a bomb which exploded at a road construction site near the Basque town of Orio on Monday.

"The Interior Ministry reminds Ertzaintza (local police) agents that there are 20 terrorist actions as yet unsolved and, as was demonstrated by the recent attack in Orio, part of the structure of freed and legal (unknown) collaborators has been able to remain intact," the message to agents said.