The six groups, representing mainly nationalist families, said current " investigatory, prosecutorial and judicial arrangements offer no realistic prospect of truth recovery for bereaved families".

"We believe that the only way to bring truth to the greatest number of families is through an international independent truth commission," the groups said in a statement.

The groups - the Pat Finucane Centre, Relatives for Justice, Justice for the Forgotten, Ardoyne Commemoration Project, An Fhirinne, and Firinne Fermanagh - have been discussing the issue among themselves for the past year.

Their intervention comes as the Eames-Bradley group, which the Government has asked to make recommendations on dealing with the past, is conducting public meetings.

The joint statement said the commissioner "should be available as a mechanism for all victims who wish to have their cases investigated thoroughly".

"The focus of such a commission should be on truth and acknowledgement rather than prosecutions," it said.

"The criminal justice system has frustrated rather than facilitated access to the truth."

The organisations said "all combatant groups, British, republican and loyalist should cooperate in good faith and have a moral duty to do so."

They said a commission should "establish the causes, context and consequences of the conflict", and also the policies and actions of the British and Irish governments should be examined.

"We believe that an independent international truth commission provides the best opportunity for truth recovery for the greatest number of those affected by the conflict," their statement said.

"We believe this will contribute to individual and societal healing and recovery, dealing with the legacy of the past in a positive way and building a better future for everyone."

Sources close to the Eames-Bradley group say only "small numbers" of the groups they have met so far have argued for a full truth commission.

But one source said a presentation by Relatives for Justice had made a " powerful" argument for a full truth commission.