Simon Carswell, Washington Correspondent

Northern Ireland still requires "urgent work" and faces "more tests to come", US president Barack Obama has said, ahead of the 15th anniversary of the Belfast Agreement next month.

Meanwhile, former British prime minister Tony Blair said the situations is “still fragile and difficult” but that the “gains of peace are visible and clear”.

In a statement issued by the White House yesterday evening, Mr Obama said that the people of Northern Ireland and their leaders had "travelled a great distance over the past 15 years" and "traded bullets for ballots, destruction and division for dialogue and institutions, and pointed the way toward a shared future for all."

The president warned of further challenges ahead.

“There are still those few who prefer to look backward rather than forward – who prefer to inspire hate rather than hope. The many who have brought Northern Ireland this far must keep rejecting their call,” he said.

Every citizen and political party “needs to work together in service of true and lasting peace and prosperity,” he said. He promised that the US would continue to support the people and political leaders of Northern Ireland.

“I pledge our continued support for their efforts to build a strong society, a vibrant economy and an enduring peace,” he said.

Mr Obama will visit Northern Ireland in June when he attends the G8 summit of world leaders in Fermanagh. The president said he would reaffirm America's support for Northern Ireland during that visit.

US secretary of state John Kerry said in a separate statement that the progress made in Northern Ireland was "significant and inspiring" but the promise foreseen in the agreement was "incomplete."

He described the 15th anniversary as “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.

“A spirit of cooperation and an unwavering commitment to the rule of law are essential to achieving these goals and a necessary condition for unlocking the full economic potential of Northern Ireland.”

The 65-page Belfast Agreement was signed on April 10th 1998, establishing the Northern Ireland Assembly and a political framework to advance the Northern Irish peace process.

Mr Obama discussed the peace process with Taoiseach Enda Kenny and Northern Ireland First Minister Peter Robinson and Deputy First Minister Martin McGuinness at meetings during the official St Patrick's Day celebrations in Washington last week.

This morning, Mr Blair said he looked back on the Belfast Agreement with " a lot of pride and a lot of gratitude to the people who helped make it happen".

It took some time after the agreement for the effects of it to be reflected in the setting up of a stable Government but there had been much progress since then. "Even then frankly if you had told me that Martin McGuinness and Ian Paisley would be sitting around the same table together I would have been pretty doubtful," he told RTÉ's John Murray Show .

“From time to time my mind and spirit often returns to Northern Ireland and what happened there and I just hope and pray it cotinues.”

He occasionally still sees former taosieach Bertie Ahern, another key figure in brokering in the deal. "We remain good friends," he said.

Mr Blair said his infamous remark after the agreement was signed about feeling “the hand of history” on his shoulder was “one of the more crass comments” he made during his career.

“I literally didn’t have it in my mind to do a soundbite ... and then the hand of history popped into my mind and out it came from my mouth and there it was,” he said.