The UK’s MI5 intelligence service on Tuesday raised the terrorism threat level in Northern Ireland’ to “severe” amid an increase in activity by dissident Irish republican militants.

Severe is the second-highest rung on a five-point scale and means an attack is considered highly likely. The threat had stood one notch lower at “substantial” for the past year.

Paramilitary groups in Northern Ireland laid down their arms after the 1998 Good Friday peace accord largely ended three decades of violent conflict between Irish republican and British loyalist groups and UK security forces that killed more than 3600 people. But small Irish Republican Army splinter groups have continued to launch sporadic attacks on security forces.

UK Northern Ireland Secretary Chris Heaton-Harris said that “in recent months, we have seen an increase in levels of activity relating to Northern Ireland-related terrorism, which has targeted police officers serving their communities and also put at risk the lives of children and other members of the public”.

Secretary of State for Northern Ireland Chris Heaton-Harris arrives for a Cabinet meeting at 10 Downing Street in London. Photo / AP

He urged people to “remain vigilant but not be alarmed.”

Last month, senior police officer John Caldwell was shot by two masked men as he coached a children’s soccer team in the town of Omagh, nearly 100km west of Belfast. Police say he suffered life-changing injuries.

An IRA splinter group known as the New IRA claimed responsibility.

The change to the threat level comes ahead of events commemorating the 25th anniversary of the Good Friday agreement.

US President Joe Biden is expected to visit Northern Ireland to mark the anniversary. Major players in the peace process, including former US President Bill Clinton and ex-British Prime Minister Tony Blair, are also due in Belfast for commemorative events.

The threat level for the rest of the UK remains at substantial, meaning an attack is considered likely.