Masked ETA members announced the cease-fire in a video distributed to Spanish media, and ETA's statement also appeared on the website of the pro-independence Basque newspaper Gara, which often serves as an ETA mouthpiece.
But the statement made no mention of ETA dissolving or giving up its weapons - key demands from successive Spanish governments. And a previous ceasefire that ETA declared in 2006 and called permanent ended after only nine months.
Interior Minister Alfredo Perez Rubalcaba said Spanish governments and mainstream Spanish political parties have maintained that all they want to hear from ETA is that it is disarming and giving up.
"It is evident that once again today, ETA has not done what we democratic parties expected," he said in a brief appearance before reporters. He took no questions.
Europe's last major violent political militant group declared a cease-fire in September but gave no details about how long it would last. The statement issued Monday specified that the group now supports a "permanent and general cease-fire which will be verifiable by the international community."
It added: "This is ETA's firm commitment toward a process to achieve a lasting resolution and toward an end to the armed confrontation."
ETA said in Monday's statement it is open to dialogue and negotiation but it also reiterated standard ETA positions, such as its belief that the Basque people have the right to decide whether to remain part of Spain or break away.
Perez Rubalcaba said ETA had maintained a catalogue of demands and an arrogant tone. "In other words, ETA still wants a price to be paid for ending violence," he said.
The minister added: "If you ask me if this is the end (of ETA), I would say no. If you ask me if this what Spanish society hoped for, I would say definitely not. Put another way, is this bad news? It is not. But it is not THE announcement."
Kepa Aulestia, a former ETA member who now works as a political commentator and journalist in the Basque region, agreed that ETA is resisting giving up without some kind of concession from a government that seems determined not to make any, after getting burned by negotiating with ETA in 2006 only to see that year end with a huge ETA car bombing that killed two at Madrid airport. He called ETA extremely weak after years of arrests and dwindling support at the grassroots level.
"We are witnessing a tug of war that ETA is trying to maintain to the effect that it is not yet ready to go away definitively," Aulestia said from Bilbao, the region's main city.
ETA is considered a terrorist organization by Spain, the European Union and the United States. It has killed more than 825 people since the late 1960s, but has recently been decimated by arrests and dwindling support.
Its outlawed political wing, Batasuna, wants to create a new party that rejects violence and turns its leaders into legitimate politicians. Batasuna, which was banned in 2003, is now backed by some mainstream Basque parties and civic groups, and has become increasingly vocal in its new position that blowing up police cars and shooting politicians is hindering Basques' cherished but unlikely goal of a country of their own.
In Spain, speculation has been rife for weeks that ETA would issue a new statement, but the government had urged caution, saying the group has raised hopes before only to dash them.
Ex-Batasuna leaders say they want their party to have a new voice in the small but wealthy region of northern Spain, a proud patch that boasts its own ancient language and culture and already enjoys a broad degree of self-rule.
ETA's political supporters hope to field candidates for Basque municipal elections scheduled for May of this year, and face politicial and financial oblivion if they are shut out.
Rubalcaba said that in order for Batasuna to field candidates, either ETA has to dissolve or Batasuna has to renounce links with ETA. He said neither of these has happened.
Now, the key thing to watch is whether Batasuna takes steps on its own to regain legal status. If it does, this could cause a rupture with ETA over the mid- to long-term, Aulestia said.
ETA declared what it called a permanent cease-fire in 2006 - after the government had said it would bet willing to negotiate it if did - but that truce ended up lasting just nine months as talks with the government went nowhere. ETA resorted to violence in December 2006 with the car bombing that killed two people at Madrid's Barajas airport.
ETA's last deadly attack in Spain was a July 2009 car bomb that killed two policemen on the Mediterannean island of Mallorca.