The Basque militant group ETA has announced it has "completely dissolved all its structures", according to a letter sent to Basque institutions and civil society groups.

In the letter, published by Spanish online newspaper eldiario.es on Wednesday, ETA said it acknowledges its responsibility in failing to solve the Basque "political conflict".

ETA, whose initials stand for "Euskadi ta Askatasuna" - or "Basque Homeland and Freedom" - killed 853 people in its armed campaign to create an independent Basque state in northern Spain and southern France.

Founded in the midst of General Francisco Franco's regime, the group grabbed global headlines when it killed the dictator's anointed successor, prime minister Luis Carrero Blanco, in 1973.

The decision, ETA said in the letter, "doesn't overcome the conflict that the Basque Country maintains with Spain and with France".

It added: "The Basque Country is now before a new opportunity to finally close the conflict and build a collective future.

"Let's not repeat the errors, let's not allow for problems to rot."

Responding to the announcement, Spanish interior minister Juan Ignacio Zoido vowed to keep investigating unresolved crimes attributed to ETA. He said police will "continue to pursue the terrorists, wherever they may be".

"ETA obtained nothing through its promise to stop killing, and it will obtain nothing by announcing what they call dissolution," he told reporters.

The group's bloodiest period came as Spain transitioned from dictatorship to democracy in the early 1980s, targeting not only members of the military and police forces but also politicians, entrepreneurs, civilians and some of its own repentant militants who wanted to leave.

At least 60 other people were also killed at the hands of death squads set up by members of Spain's security forces.