Families of the Disappeared is demanding that the IRA declare an amnesty for people coming forward with information about the location of the remains of their loved ones.
The pressure group said that without such a promise many people would be too frightened to disclose what they knew about where the bodies are buried.
The Sinn Fein chairman, Mr Mitchel McLaughlin, has urged anyone with information to disclose it "as a matter of urgency".
Mr McLaughlin was speaking after an impassioned plead by the co-chairman of the North's Victims Commission, Sir Kenneth Bloomfield, for paramilitary organisations to "come clean" and identify the secret burial sites of people abducted and killed during the Troubles.
The IRA is believed to have been involved in nearly all the cases. However, Mr Seamus McKendry of Families of the Disappeared, whose mother-in-law, Jean, is one of those missing, yesterday described Sinn Fein's response as inadequate and questioned its sincerity.
Mr McKendry said that if Sinn Fein was serious about helping the families it would urge the IRA to declare an amnesty for anyone with information. "Many people are frightened to come forward," he said. "Only a few people know the locations of the bodies so the IRA could easily find out the identity of anyone who passed on information.
"Even former IRA members would fear for their lives if they helped us. They think they would be killed, injured or put out of the country. Sinn Fein should call on their military wing to take the necessary steps to remove this fear.
"If the IRA declared an amnesty, I would hope that people would come forward within the next few days and the families could be finally put out of their anguish."