MADRID (AFP) - Three explosions, two blamed on ETA separatists, shook the Basque country in northern Spain early Thursday, damaging buildings but causing no casualties, the Basque autonomous police told AFP.

The first "powerful" blast came around midnight (2200 GMT Wednesday) in the Vizcaya town of Arrigoriaga "in a building belonging to the Spanish labour ministry," the police said.

The cause of the blast was not immediately clear.

Two bombs went off shortly afterwards near the Basque institute of work health and safety in San Sebastian, causing "slight damage" after a phone call from armed Basque separatist group ETA to regional traffic department DYA warning of imminent blasts, the police added.

Calls to DYA have traditionally been used by ETA to warn of imminent explosions in the Basque country.

The police, who were investigating the first blast, said it apparently had "nothing to do" with the other two.

The explosions came hours after the jailing of the mayor of Mondragon in the Basque country where a former Socialist councillor had been assassinated by ETA on March 7, two days before the Spanish elections.

Innocencia Galparaso, a member of the Basque Nationalist Action party (ANV) who refused to condemn the murder, was sent to prison by anti-terrorist judge Baltasar Garzon for "collaboration with a terrorist organisation."

The last explosion by ETA was on April 20 when a bomb went off outside a community centre in the northern Spanish town of Elgoibar, causing damage but no injuries.

The blast, which occurred at the Casa del Pueblo or House of the People, also caused significant damage to surrounding buildings, including the local offices of Spain's ruling Socialist Party.

ETA has killed over 800 people in Spain in shootings and bombings as part of its 40-year campaign for an independent Basque homeland.

It has killed two police officers and a former Socialist Party town councillor in shootings since officially calling off a ceasefire in June 2007 citing a lack of concessions on the part of the government in peace talks.

Socialist Prime Minister Jose Luis Rodriguez Zapatero, who was re-elected last month in a general election with a slightly bigger majority, launched the talks in June 2006, three months after ETA announced its truce.

Since the end of the ceasefire, Spanish authorities have adopted a hard line, arresting dozens of members of both ETA and its banned political wing, Batasuna, and Zapatero has ruled out any further talks with the group.

Zapatero said on Monday that he believes a cross-party strategy to fight ETA is possible during his second term.

"It would be very convenient to have a certain understanding, that we set reasonable and limited objectives, among all political forces regarding the fight against ETA and against terrorism," he told public television TVE.

"I think it is possible, we have to work for it and of course we all have to have a responsible attitude," he added in his first interview since being sworn in earlier April for his second term as prime minister.

Zapatero proposed the cross-party strategy to fight ETA on April 8 in his first speech to parliament since he was re-elected in a March 9 general election with a slightly bigger majority in the assembly.

His first four-year term was marked by confrontation with the main opposition conservative Popular Party over ETA. The PP vocally opposed negotiations with ETA launched by Zapatero after the group announced its ceasefire in March 2006.

ETA, whose initials stand for Euskadi ta Askatasuna, or Basque Homeland and Freedom in the Basque language, is designated a terrorist organisation by the United States and the European Union.