Spanish Prime Minister Mariano Rajoy is calling the disbanding of ETA "noise and propaganda," vowing to carry on prosecuting the militants in the Basque separatist organization.

Speaking a day after an ETA letter emerged stating that it had "completely dissolved its structures," Rajoy said the group had failed in imposing through violence an independent Basque state.

A final public declaration from ETA, which Spain regards as a terrorist group, is expected later on Thursday.

"Whatever ETA does or says, it won't find any loophole for impunity," Rajoy said. "ETA can announce its disappearance, but its crimes or the action of the judiciary won't disappear."

ETA — which stands for "Basque Homeland and Freedom" — killed 853 people, most of them after Spain transitioned to democracy in the late '70s.