Britain's MI5 intelligence agency has increased the threat level from domestic terrorism to Northern Ireland to "severe" - meaning an attack is highly likely, Britain said on Tuesday.

Chris Heaton-Harris, the British minister for the province, said in a written ministerial statement that the level had been increased from "substantial" following the recent targeting of police officers.

An off-duty officer was seriously injured in a gun attack last month, in an incident condemned by the governments in Dublin and London.

"The public should remain vigilant, but not be alarmed, and continue to report any concerns they have to the Police Service of Northern Ireland," he said.

The change comes roughly a year after Britain lowered the threat level for the province to "substantial" for the first time in more than a decade.

It also comes ahead of next month's 25th anniversary of the Good Friday Agreement peace deal that largely ended the "Troubles," three decades of violence that had convulsed Northern Ireland since the late 1960s.

Earlier this month U.S. President Joe Biden said he had accepted an invitation by British Prime Minister Rishi Sunak to visit Northern Ireland, and both Bill and Hilary Clinton are due to attend anniversary-related events.