A judge in El Salvador has reopened the case of a massacre allegedly carried out by soldiers in 1981 that is considered the worst atrocity committed during the country's brutal civil war, a lawyer involved in the matter said on Saturday.

The massacre took place in the northeastern town of El Mozote, allegedly at the hands of an elite army unit, and resulted in the deaths of between 900 and 1,200 people, mostly women and children.

The decision by Judge Jorge Alberto Guzman to reopen the matter marks the first time such a case has been allowed since an amnesty law was declared unconstitutional in July.

The law sought to absolve militants on both sides of the conflict who took part in war crimes, and was often invoked by judges to explain why they could not hear such cases.

"The reopening of the case is an open door to seek justice that has been denied for so many years to victims of crimes against humanity in El Salvador," lawyer Ovidio Gonzalez told Reuters.

The reopening of the case follows formal complaints by local human rights groups and victims families seeking justice for those killed in El Mozote.

Those pushing the case seek a truthful accounting of what happened and the facts surrounding who ordered the massacre, Gonzalez said. They are not seeking punishments such as jail terms, he added, but want those responsible to admit their roles and ask for forgiveness.

Judge Guzman has ordered military records from the time of the massacre to be turned over, as well as additional records on 14 named ex-army and security officials.

The first public hearing is not expected for several months.

While victims' families pushed for a trial as early as 1990, the case has never been heard locally. In 2010, the human rights commission of the Organization of American States recommended that El Salvador repeal the amnesty law as a means for those responsible for the massacre to be held to account.

El Salvador's civil war stretched from 1980 to 1992, taking around 75,000 lives and leaving another 8,000 people missing.

A truth commission created by the United Nations in 1992 published a report that declared the El Mozote massacre the worst war crime perpetrated during the conflict.

El Salvador's government denied for years having perpetrated the slaughter, but in 2012 the government of then-President Mauricio Funes acknowledged the state's role and apologized to the families of the victims.