The UK government Tuesday raised the terrorism threat level in Northern Ireland to "severe", ahead of an expected visit by US President Joe Biden to mark the 25th anniversary of a landmark peace accord.

The domestic spy agency MI5 now judged that the threat of an attack had gone from "substantial" to "severe", meaning an incident is judged "highly likely", Northern Ireland Secretary Chris Heaton-Harris said.

After suffering three decades of bloody unrest, Northern Ireland stands transformed since the Good Friday Agreement was adopted on April 10, 1998.

"However, a small number of people remain determined to cause harm to our communities through acts of politically motivated violence," Heaton-Harris told parliament.

He urged the public to "remain vigilant, but not be alarmed" at the announcement, which comes after a police officer was shot and seriously wounded in front of his son.

Heaton-Harris drew no link to the planned trip by Biden, who intends to visit Ireland and Northern Ireland.

The trip is expected to take in the anniversary of the peace deal, which was brokered with US mediation.

The anniversary comes with the territory locked in political paralysis, as pro-UK unionists refuse to re-enter government in objection to a post-Brexit trade deal agreed by London and the European Union.

More extreme militants in the pro-UK camp have issued periodic warnings about the consequences of the deal, which they argue leaves Northern Ireland at risk of political and economic divorce from Great Britain.

But Heaton-Harris said that sustained bloodshed was a thing of the past.

"The political future of Northern Ireland rests with the democratic will of the people and not the violent actions of the few," he said.

"Together we will ensure there is no return to the violence of the past."