The threat level from domestic terror in Northern Ireland has been raised from "substantial" to "severe," the UK's Northern Ireland Minister Christ Heaton-Harris said on Tuesday.

The change in threat level comes after the shooting and injury of an off-duty police officer in the Northern Irish city of Omagh in February.

"The public should remain vigilant, but not be alarmed, and continue to report any concerns they have," Heaton-Harris wrote in a statement.

It comes ahead of a planned visit by US President Joe Biden to the Republic of Ireland and Northern Ireland expected to take place next month on the anniversary of the Good Friday peace agreement.

Brexit and the border issue

Tensions have risen in Northern Ireland since the UK departed the EU, leaving the territory in a state of limbo.

As part of the Good Friday agreement, there can be no hard border on the island of Ireland, but Brexit has meant the introduction of border checks on goods going between the UK and the EU.

The current solution has been to carry out border checks on goods crossing the Irish Sea between Northern Ireland and the UK mainland, but this has angered pro-UK groups who fear it is weakening the union.

The shooting of the officer last month, which was claimed by a republican group known as the New IRA, has sparked fears of escalating violence and a return to the so-called Troubles — an era of widespread sectarian violence that was brought to an end by the Good Friday agreement.

'No return to the violence of the past'

But the minister has tried to assure people that the threat of violence will not sway civic life.

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

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

He made no reference to Biden's planned visit. Leaders are set to meet in Belfast on April 10 to mark the anniversary of the Good Friday agreement which was in part brokered by the US under then-President Bill Clinton.

The Northern Ireland threat level was first introduced in 2010 and was already set at "severe." It was only downgraded to substantial in 2022.

ab/jcg (Reuters, AP, AFP)