"The State must assume a leadership role and engage more actively to respond to the demands of thousands of families searching for the fate or whereabouts of their loved ones, who disappeared during the civil war and the dictatorship", said the experts of the Working Group of the United Nations on Enforced or Involuntary Disappearances.
The comments came at the end of a visit by the UN working group to Spain to investigate alleged mass forced disappearances under the regime of Francisco Franco.
Franco ruled the country from the end of the civil war in 1939 to his death in 1975.
When Spain made its delicate transition to democracy after the dictator's death, political groups that forged its new constitution agreed on an amnesty for political prisoners that also served to protect officials from score-settling.
It was seen as a necessity by the leaders tasked with unifying the country still smarting from the wounds of the 1936–1939 civil war.
But it has never satisfied the victims and their families, nor those who believe that international laws against the worst human rights violations should trump the national amnesty.
"It is regrettable the situation of impunity for cases of enforced disappearances that occurred during the civil war and the dictatorship," said the working group in a statement.
"There is no ongoing effective criminal investigation nor any person convicted."
The experts stressed procedural rights to an investigation, to truth and to justice were central to victims' perceptions of reparation.
In a separate interview, working group member Ariel Dulitzky said the advanced age of many of the relatives and witnesses meant action on the issue was urgently required.
The UN also pointed out the "limited scope" of Spain's 2007 Law of Historical Memory and "the lack of budget for its implementation".
This initiative recognized victims on both sides of Spain's civil war and formally condemned the Franco regime.
The UN working group said, however, that the continued existence of the Amnesty Law limited the use of the Law of Historical Memory .
There was also an "absence of an autonomous crime of enforced disappearance", and a "lack of a law on access to information", the experts said.
On top of this, there were "difficulties in accessing archives and the lack of a national plan for the search of disappeared persons," the group argued.
"Since the return of democracy, Spain has taken limited steps to ensure truth, justice, reparation and memory for the cases of enforced disappearances committed during the civil war and the dictatorship," the experts noted.
"This progress has been achieved through initiatives, many of which have been led or carried out mainly or exclusively by the relatives of the victims, or by civil society and some State institutions, mainly in certain Autonomous Communities.
"The State should assume its responsibility to ensure that these initiatives are part of a comprehensive, consistent and permanent State policy," they added.
The news comes after an Argentinian judge's recent issuing of arrest warrants for four officials who served under Franco.