Basque separatist group ETA must show it can keep its side of the deal in its peace process with the Spanish government, Prime Minister Jose Luis Zapatero said in an interview with a French newspaper published today.
The government agreed to hold peace talks with ETA after it announced a permanent ceasefire in March.
But the process has been marred by street-fighting incidents that have the hallmarks of ETA and the government has yet to confirm that the talks have actually begun.
Asked by Le Figaronewspaper what interest ETA could have in giving up its weapons, he said: "I cannot judge their intentions. We have kept our side of the bargain. The ball is in ETA's court."
"If the terrorist group wants to take some steps forwards, the government will take some steps forward. If it wants to get lost in the tunnel of history, we will find ourselves back in a situation that is undesirable for everybody."
He said finding peace could take some time.
ETA has killed 800 people in four decades of armed struggle for independence for the Basque region of northern Spain.
ETA threatened to break off the peace process earlier this month and also said it would begin a "new phase of struggle against the French government".
ETA wants part of southwest France to become part of an independent Basque Country but Paris has refused to participate in peace talks.
"It would be absurd to get France mixed up in this," Mr Zapatero said.
He also said he did not see any divisions in ETA.
"ETA is very unified from a decision-making point of view," he said.
"There may be some nuances but we do not have enough proof to think that opposing views exist in the organisation."