The armed Basque separatist group ETA called off its 15-month-old cease-fire in a statement Tuesday, formalizing what many saw as the demise of a once-promising peace process already struck down by a deadly bombing in December.

ETA said the truce it called in March 2006 would end at midnight Tuesday, and that after that it would be "active on all fronts to defend the Basque homeland." The move could mean a quick resumption of violence, and authorities say they are on high alert.

In a statement sent to two Basque newspapers, it blamed the government — and particularly Prime Minister Jose Luis Rodriguez Zapatero — for the failed peace process, accusing the Socialist leader of becoming a fascist.

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"Zapatero's character has turned into a fascism that left parties and citizens without rights," ETA said.

It was a bitter pill for Zapatero, who had made ending the nearly 40-year-old conflict one of the main goals of his administration. The prime minister will address the nation later Tuesday, his office said.

The government had feared such a move by ETA might be imminent since it decided not to allow ETA's outlawed political wing, Batasuna, and associated political parties from running in May 27 local elections. Officials said they were aware the move could embolden ETA hard-liners wary of the peace process, but felt they had no choice in the wake of the Dec. 30 bombing of a Madrid airport parking garage that killed two people.

The pro-independence movement had called the elections a key part of the peace process.

In its statement Tuesday, ETA contended that the Basque leg of the elections were illegitimate without Batasuna, and complained that the Spanish judiciary system had continued to crack down on ETA members despite the cease-fire. In the past, it has suggested the government committed itself to going easy on the pro-independence movement while negotiations were under way.

Now, "minimum democratic conditions for a negotiating process do not exist," ETA said in the statement sent to the pro-independence newspapers Berria and Gara.

While ETA's announcement was ominous, most of Spain had considered the cease-fire over when the group blew up the airport parking garage. As rescue workers combed through the rubble, ETA insisted in a bizarre statement that the truce was still in place, and indicated the two fatalities were unintended.

When ETA originally declared the cease-fire, it called the truce "permanent" and said it wanted a negotiated end to the nearly 40-year conflict, which has left more than 800 people dead and made ETA Europe's last armed political militancy group.

Zapatero announced in June 2006 that his government would negotiate with ETA, earning the wrath of the conservative opposition, who accused him of giving in to terrorism.

The two sides met a number of times, at least once at an undisclosed location outside of Spain. But the talks made no significant progress and Zapatero's government said after the December bombing that it would not have contact with ETA.

Over the past few months, signs have been growing that ETA was reactivating.

The group warned in a series of communiques that it was not pleased with how the peace process was going.

In March, police arrested about a dozen suspects believed to be part of a newly formed ETA cell. And business people in the Basque region say they recently started receiving extortion letters from ETA, a traditional ETA fundraising method.

On Monday, just hours before the end of the cease-fire was announced, police were quoted by the newspaper El Pais as saying they feared a resumption of ETA violence and had warned the government to that effect. Police told the paper ETA might stage a major attack aimed at causing damage but not fatalities — much like what it said it had tried to do with the airport bombing.

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