Northern Ireland must "consolidate the gains" of peace, says the US secretary of state, ahead of the 15th anniversary of the Good Friday Agreement.
John Kerry, who replaced Hillary Clinton, said progress was "significant and inspiring" but the promise envisaged by the deal was "incomplete".
He said Friday was a "call to action".
The 10 April 1998 agreement ended 30 years of conflict and enshrined a peaceful framework for tackling religious and political divisions.
In a statement, Mr Kerry congratulated the people of Northern Ireland.
The "courage, conviction, and hard work of leaders and communities over the past 15 years... have led to a more peaceful and vibrant Northern Ireland", he said.
He went on: "The progress that has been made is significant and inspiring, but the promise envisioned by the agreement is incomplete. The 15th anniversary of the Good Friday Agreement is a call to action to consolidate the gains of the last 15 years.
"This is an appropriate moment for all parties to rededicate themselves to achieving a shared future and to healing the divisions of the past."
He said the US remained committed to working with all parties to secure a "stable, peaceful, and prosperous future".
The break-up of Ireland in 1921, in which the south became a separate state known as the Republic of Ireland, led to decades of unrest and violence in Northern Ireland, which remained part of the UK.
Nationalists believe the north should join a united, independent Ireland, while unionists or loyalists, think Northern Ireland should stay as part of the UK.
The period known as the Troubles began in the late 1960s and lasted for nearly 30 years, with thousands on both sides killed.
The Good Friday Agreement was seen as a major step forward in the peace process.
The 65-page document, signed in Belfast in 1998, led to the Northern Ireland assembly and executive being set up, new cross-border institutions involving the Irish Republic and a body linking devolved assemblies across the UK with Westminster and Dublin.
The Irish Republic also dropped its constitutional claim to the six counties which form Northern Ireland.
Several spates of violence in Northern Ireland this year have prompted warnings by leading politicians that dissident extremists will not be allowed to disturb the hard-won peace.