The Basque militant group ETA has offered an unprecedented apology for the pain caused during its armed campaign for independence from Spain and France, and vowed not to return to violence.

ETA, which killed around 850 people including police, politicians and entrepreneurs over four decades, is due to announce its final dissolution early next month, ending one of Europe’s last standing violent nationalist conflicts.

After nearly half a century of car bomb attacks, shootings and kidnappings, the group gave up its violent campaign in 2011. One year ago, the organisation also handed over most of its remaining arsenal to authorities.

In a statement published by Basque newspapers Berria and Gara, ETA acknowledged its responsibility for the pain caused by assassinations, torture, kidnappings and people forced to leave the Basque country, in a vague reference not only to ETA’s victims but also to the plight of some of its own militants.

“We want to show our respect to the dead, the injured and the victims that ETA’s actions have caused,” the statement said.

“We really are sorry.”

Spain’s government, which considers ETA a terrorist organisation, welcomed the move but said the apology came too late.

Prime Minister Mariano Rajoy’s government said: “ETA should have sincerely and unconditionally asked for forgiveness for the damage caused a long time ago.”

It added that Friday’s announcement was “nothing more than another consequence of the fortitude of the rule of law that has defeated ETA with the arms of democracy”.

ETA’s victims were also critical of the announcement because it sought the forgiveness of victims “who didn’t have a direct participation in the conflict” – apparently excluding those who had been specifically targeted by ETA.

AVT, a national association of terrorism victims, said the statement aimed to “whiten” ETA’s past, while COVITE, another victims’ group based in the Basque town of San Sebastian, said the distinction between “guilty and innocent victims” treats them “as collateral damage in the imposition of a totalitarian project”.

ETA emerged in the late 1950s during the dictatorship of Gen Francisco Franco with the stated aim of forming an independent state from Basque areas on both sides of the Pyrenees. Basques have a distinct culture and an ancient language, Euskara.

In the 80s, shadowy death squads killed and tortured dozens of ETA militants in what was known as the Spanish government’s “dirty war” against the group.

The official Basque regional broadcaster ETB, which has in the past had access to the organisation’s plans, reported this week that ETA’s dissolution would be announced in the first weekend of May.

ETB said that an event to mark the end of ETA – a Basque-language acronym for Euskadi Ta Askatasuna, meaning Basque Homeland and Freedom – will be held in southern France before the final announcement by the group.

The group said it is committed “to finally overcome the consequences of the conflict and not to fall into its repetition”, adding that “this political and historical conflict should have had a fair and democratic solution a long time ago”.