Ms Nelson had been identified by human rights groups as being at only "low-grade" risk in Northern Ireland, said Ms Mowlam. She had not wanted protection from the RUC.
Ms Mowlam said that she believed a way out of the current impasse over the new Assembly was possible, and that Northern Ireland was "a hair's breadth away" from a deal that would unblock progress on the new executive.
British officials said that hopes were rising that a deal would be made following talks today in Washington between the leaders of the Northern Ireland political parties and President Bill Clinton.
Ms Mowlam, Bertie Ahern, the Irish Prime Minister, David Trimble, the First Minister, and Gerry Adams, leader of Sinn Fein, will all be in Washington for the talks.
"We are very far along this road. It would be a tragedy if we didn't keep on walking down it," said Ms Mowlam. "A gesture ... a beginning" was needed to end the stalemate between the nationalists and loyalists over whether Sinn Fein could take its place in the executive before decommissioning began. "Everybody is going to have to move, everybody is going to have to do something," she said.
David Trimble said that he hoped President Clinton would ask Gerry Adams to begin decommissioning. "The important thing that can be done by people here in Washington is to use what influence they have with those elements in Northern Ireland who have yet to begin their end of implementing the agreement," he said.
Sinn Fein are playing a "long game" he added, saying that he feared that progress on decommissioning might be stretched out for weeks if not months. He also said he was concerned that the issue of decommissioning might become entangled with the forthcoming report from Chris Patten, the former governor of Hong Kong, on policing in Northern Ireland.
US officials say that Mr Clinton will reiterate that all parties must respect the spirit as well as the letter of the Good Friday Agreement, and that some movement from Sinn Fein is necessary.
Seamus Mallon of the SDLP has outlined a formula that would see further British military withdrawals and demilitarisation, efforts to build confidence by identifying the burial sites of some of Northern Ireland's many "disappeared", and a clear commitment to begin disarmament.
But the key would be a change of tack from Sinn Fein.
"I think there's room in there for Sinn Fein to move," he said.Reuse content