Republicans have not given up hope of resolving the "Disappeared" issue, Sinn Fein's Martin McGuinness has said.
A series of searches have taken place to locate the Disappeared
DUP leader Ian Paisley is to meet the mother of Columba McVeigh, murdered in 1975 but whose body remains missing.
In 1999, the IRA offered to help locate the bodies of the nine Disappeared. Three victims were found in 1999 while one was uncovered in 2003.
Mr McGuinness said his party was "not satisfied" with this and would continue striving to help the victims' families.
"We have been partially successful in that there have been a number of cases resolved as a result of the endeavours of republicans," he said.
"But we are not satisfied with that.
"We want to continue to appeal to people to help in whatever way they can to ensure that families like the McVeighs can find some comfort after the terrible ordeal they have been through over the course of many decades."
The so-called Disappeared are nine people murdered by the IRA during the Troubles whose bodies were not found for decades. Five remain undetected.
In October 2003, the IRA apologised for the grief suffered by their families.
On Monday, the mother of IRA murder victim Columba McVeigh said she appreciated the help of Mr Paisley who wants to help pursue the search for her son's body.
The 17-year-old from Donaghmore in County Tyrone was kidnapped, killed and secretly buried in 1975.
It is understood a meeting will take place between the DUP leader and the McVeigh family this week.
In 2003, Mrs McVeigh said she had given up hope of ever finding her son's remains, after a 14-day police excavation at bogland in County Monaghan turned up nothing.
The operation was the third dig in the area and followed new information passed to the Irish Government by the IRA.