"The Good Friday Agreement is the work of genius that's applicable if you care at all about preserving democracy," Clinton said of the deal signed on April 10, 1998, marking the end to a conflict lasting three decades.

He was joined in Belfast by other political leaders at the time, including the then UK prime minister Tony Blair and his former Irish counterpart Bertie Ahern.

Clinton described the agreement as a "precious gift" and said those behind it "inspired the world".

"These people gave you a gift... People who lost loved ones and bore scars; the women's groups; everybody, they gave you a gift. Make the most of it," he said.

Despite the Good Friday Agreement putting an end to "The Troubles", in which more than 3,500 people were killed, there are concerns over the future of the British province.

The power-sharing executive established in Belfast as part of the accord broke down more than a year ago and the two sides remain divided over issues including the Irish language.

"The Good Friday Agreement couldn't solve all the issues," remarked Gerry Adams, former long-time leader of the Irish republican party Sinn Fein.

Britain's decision to leave the European Union has further called into question one of the key outcomes of the deal -- the dismantling of the border infrastructure between the British province and Ireland.

While Blair praised former foes for choosing "not to forget the past, but not let it decide the future," the ex-premier voiced his concerns over Brexit.

Imposing a hard border following the EU exit "would be a disaster for the Agreement and for the relationship between the Republic (of Ireland) and the UK, and therefore for the people of Northern Ireland," he said.