Pope Francis said on Monday he was praying that the Good Friday agreement signed 25 years ago that largely ended violence in Northern Ireland can be "consolidated" to benefit the people of all of Ireland.

Francis made the comment about the 1998 agreement, which has come under strain following Britain's exit from the European Union, in his noon address on Easter Monday to tens of thousands of people in St. Peter's Square.

"Today marks the 25th anniversary of the so-called Good Friday agreement, or of Belfast, which brought an end to the violence that for decades troubled Northern Ireland," Francis said.

Signed in Belfast on April 10, 1998, the agreement is considered one of the most significant peace deals of the late 20th Century, seeking to end three decades of sectarian strife between Catholic and Protestants known as "The Troubles" that killed more than 3,600 people. 

"In a spirit of gratitude I pray to the God of peace so that what was achieved in that historic step can be consolidated to benefit all the men and women of the island of Ireland," the head of the nearly 1.4 billion-member Roman Catholic Church said.

Apart from the effect of Britain's departure from the EU, other political crises have overshadowed this week's commemorations, which will be attended by U.S. President Joe Biden, reflecting the vital role the United States played in brokering the agreement.

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.