Spanish police have arrested nine people in raids on the offices of a Basque newspaper Egunkaria. Spanish police said the operation was continuing and a spokesman declined to comment on the number of arrests.
A witness said the headquarters of the Basque language newspaper in Bilbao were sealed off with police tape.
"This is a new operation against ETA. This time it is directed at those who, according to the judge, are instruments of ETA and alert the terrorists each time there is an operation against a terrorist cell," Spanish Justice Minister Jose Maria Michavila told state radio.
The swoop is the third in as many days which has led to multiple arrests across the northern region, home of ETA, which has killed more than 800 people in a bombing and shooting campaign since 1968 to back its demands for an independent Basque state.
National police arrested at least 13 people suspected of having ETA links in predawn raids on Wednesday. On Tuesday, Basque police arrested nine people - two or three of them minors - on suspicion of carrying out an intimidating form of street violence practised by pro-ETA youths.
ETA's main media mouthpiece, newspaper Egin, was closed down by a judge in 1998 but a number of other pro-separatist publications have sprung up in its place.
Officials are accusing the newspaper of tipping off the violent Basque separatist group ETA of police movements.
Egunkariais seen as more independent than the more radical Gara, which is sympathetic to Basque radicals who refuse to condemn ETA violence.