Spanish government talked to ETA
"There have been direct and indirect contacts over a short period," he said, adding he had felt obliged to seek a negotiated end to violence.
However, he added that ETA had made unacceptable political demands.
Zapatero broke off the talks after an ETA car bomb killed two men in a car park at Madrid airport last December.
ETA said that from Wednesday it would defend the Basque country "with weapons and on all fronts".
The announcement suggests that another big attack could be imminent, observers say.
The opposition Popular Party says Zapatero's Socialist government has shown weakness by trying to deal with ETA before it renounces violence. Hitting back, Zapatero said: "I would have wished to have had the backing other prime ministers had."
He said that when he had been in opposition, he had offered then Prime Minister Jose Maria Aznar his full support.
Police were on the alert for an imminent attack, possibly against a tourist target. ----------UN's Ban condemns end to ETA's ceasefire UN Secretary-General Ban Ki-moon on Wednesday condemned a decision by Basque separatist group ETA to end their ceasefire and urged the guerrillas to reconsider, Reuters reported,
"I denounce in unequivocal terms the violence and I am quite confident that the whole international community is behind you (Spain)," Ban told reporters in Madrid after meeting Zapatero.
Ban said he was "very concerned" that an end to the truce could cost lives, and he backed the Socialist government's move to end talks after the group planted a bomb last Christmas that killed two.
"I would urge (ETA) to redouble their efforts to keep this ceasefire. Whatever the differences of opinion may be ... we need to resolve all conflict issues through peaceful means. "There cannot be dialogue when there is violence."
The rebels, who have been fighting for an independent Basque Country for since the 1960s, said they were calling off the truce because of "arrests, tortures and every type of persecution" by the Socialist government.
Zapatero said his government's approach would be one of firmness, unity and intelligence: "Firmness against any threat or violence, unity of political parties and intelligence that every day, we win more backing for peace in the Basque Country," he told the news conference.
ETA's decision to end its ceasefire means a return to a life under constant threat for hundreds of potential targets of the armed separatist group in Spain's northern Basque region, AFP reported.
"We woke up Wednesday knowing that our lives would once again radically change," the editor of weekly newspaper Cambio 16, Gorka Landaburu, told AFP.
"Checking underneath your car to make sure no bomb has been placed, no longer being able to go buy bread with ease and living with the anxiety that anyone could be a victim, is once again part of our daily lives," he added.
Landaburu is one of about 1,000 people in the region who are accompanied round the clock by bodyguards. He himself survived an attack by the armed group carried out in May 2001 at his home in the town of Zarauz.
ETA has killed more than 800 people in its four-decade campaign to set up an independent state in northern Spain and south-western France, many of them are police officers, or local and national politicians opposed to the group's separatist demands.
"I even dared to sit at cafe terraces," said the president of the local branch of right-wing Popular Party in the Basque province of Gipuzkoa, Maria Jose Usandizaga.
"But now that is finished," he added. Usandizaga is frequently cited as an ETA target and has lived under permanent police protection since 1995.
"My children have always known me to have two bodyguards, but you never get used to it, you have to give up your private life and any sort of spontaneity," she said. --------------- ETA suspects arrested in France
Three suspected members of the armed Basque group were arrested Thursday in southwestern France in the first raid since the separatists called off a 15-month-old cease-fire, officials said, AP reported.
The two men and one woman were arrested in the town of Bagneres de Bigorre, near the border with Spain, the Interior Ministry said.
A French police official, who spoke on condition of anonymity, said the detainees were believed to belong to ETA's commando network, adding that they put up no resistance.
A gun, false identity papers and equipment for stealing cars and counterfeiting registration plates were seized in a house rented by the three, Spain's Interior Ministry said.
The woman, identified as Alaitz Areitio Azpiri, 28, is suspected of being involved in recruiting ETA members and in their technical and military training, the ministry said. She has been a fugitive since 2003 and was involved in a shootout with police when she broke a roadblock in the French pilgrimage town of Lourdes in October 2004, the ministry said.
The two men were identified as Aitor Lorente Bilbao, 40, and Igor Igartua Echevarria, 37.
Lorente Bilbao went into hiding after he was released from jail in Spain in 2006 after serving seven years for arms possession. Igartua fled Spain in 2000 as police moved to break up ETA's so-called Vizcaya cell, the statement said.
The newspaper El Pais on Wednesday quoted a police report as saying ETA had about 100 active members and considerable bomb-making capacity. It has more than 700 members in jail in Spain and France.