LONDON: A quarter of century since the signing of a peace deal that largely ended violence in Northern Ireland, British Prime Minister Rishi Sunak said efforts had to be intensified to restore the power-sharing government central to the accord.

Signed in Belfast on April 10, 1998, the Good Friday agreement is considered one of the most significant peace deals of the late 20th Century, seeking to end three decades of sectarian strife that killed more than 3,600 people.

But peace has come under strain following Britain's exit from the European Union and other political crises have overshadowed this week's commemorations.

U.S. President Joe Biden will fly into Northern Ireland on Tuesday to attend events marking the 25th anniversary of the deal, a reflection of the role the United States played in brokering the agreement.

"This is an agreement born of partnership between the British and Irish governments and, as we will see from President Biden’s visit this week, it continues to enjoy huge international support from our closest allies," Sunak said in statement on Monday.

"But most importantly, it is based on compromise in Northern Ireland itself. As we look forward, we will celebrate those who took difficult decisions, accepted compromise, and showed leadership - showing bravery, perseverance, and political imagination."

Angry about post-Brexit trade rules that treated the province of Northern Ireland differently to the rest of the United Kingdom, the Democratic Unionist Party, the largest pro-British party, has boycotted the power-sharing devolved government central to the peace deal for more than a year.

Last month, Britain's MI5 intelligence agency increased the threat level in Northern Ireland from domestic terrorism to "severe" - meaning an attack was considered highly likely.

Irish Prime Minister Irish Leo Varadkar on Sunday pledged to intensify efforts with Sunak to break the political deadlock in the province. Sunak said it was a time to hail those who secured the 1998 deal and reflect on the progress since made, but also to redouble efforts.

"We stand ready to work with our partners in the Irish government and the local parties to ensure that the institutions are up and running again as soon as possible," he said. "There is work to be done."