In a statement issued to Basque country newspaper Gara on Friday, the separatist group said peace talks with the government were bogged down because of "the mean attitude of the political parties".
The statement said negotiations were at an "impasse", and accused the government of hounding Basque nationalist politicians.
"If the attacks continue against Euskal Herria, Eta will respond," the group said, without specifying what its response might be.
Euskal Herria means the Basque country in the region's ancient language, which led the group to 38 years of armed struggle in which more than 800 people were killed.
Jose Luis Rodriguez Zapatero, Spain's prime minister, said in June thaat he would begin peace talks with Eta, which wants independence for the Basque Country in northern Spain and southern France and declared a ceasefire in March.
But there has been little progress on legalising the banned Basque nationalist party Batasuna - a step which is seen as crucial if peace talks are to succeed - and a judge recently said he would stop the group from holding a pro-independence demonstration.
There was no immediate response from the Spanish government to the Eta announcement.
Eta has been seriously weakened by hundreds of arrests in recent years and polls show that only a minority of Spanish Basques want independence.