(Adds interior minister, prime minister)

By Jason Webb

MADRID, May 14 (Reuters) - A car bomb exploded at a police barracks in northern Spain's Basque Country early on Wednesday, killing one officer and wounding four others in an attack the regional government blamed on ETA militant separatists.

Prime Minister Jose Luis Rodriguez Zapatero, who broke off an attempt at peace talks with ETA in December 2006, called the attack "cowardly, miserable and criminal".

The bomb went off in a Civil Guard barracks in Legutiano, a regional police spokeswoman said, killing a 41-year-old officer, Juan Manuel Pinuel-Villalon.

Police said there was no advance warning of the blast, in which two men and two women officers were also hurt, and no one had claimed responsibility.

But the government blamed the ETA Basque separatist group which has waged four decades of violent struggle in which more than 800 people have been killed.

"They were hoping to carry out a massacre," Interior Minister Alfredo Rubalcaba told reporters after travelling to the Basque Country.

"They left a car bomb packed with a substantial amount of explosives at the door of a barracks where there were a total of 29 people, including five children and an 18-year-old girl," Rubalcaba said.

Police at the scene wore masks to protect their identities from ETA rebels as a rescue team with dogs searched for any people who could still be trapped in the rubble.

The blast blew away part of the barracks' roof, leaving broken wooden beams exposed.

The ETA wants independence for Basques, whose traditional lands lie in northern Spain and southwestern France.

Polls show most Basques do not seem to want independence, although the leader of Spain's Basque regional government Juan Jose Ibarretxe is defying the Spanish government with plans to hold a referendum on whether to begin a debate on ties with Spain.

More than 750 suspected ETA members have been arrested since 2000, and the group is believed to have been seriously weakened.

The Spanish government began peace talks with ETA after it declared a ceasefire in 2006 but broke them off after the rebels killed two people in a bomb attack at Madrid airport.

ETA's last fatal victim was a former small town politician for Spain's governing Socialist Party, gunned down just before a national election in March, police said. (Additional reporting by Raquel Castillo and Jane Barrett, Editing by Ralph Boulton)

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