MI5 has increased the terror threat level in Northern Ireland from "substantial" to "severe", meaning an attack is highly likely.
Northern Ireland Secretary Chris Heaton-Harris cited a "small number" of individuals who remain determined to use "politically motivated violence".
Announcing the change on Tuesday, he urged the public to "remain vigilant" but "not be alarmed".
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The move comes after a series of incidents targeting security forces in Northern Ireland, including the attempted murder of Detective Chief Inspector John Caldwell in Omagh last month.
The threat level for the rest of the UK remains at "substantial", meaning an attack is considered "likely".
Northern Ireland's level had been lowered to substantial a year ago - the first time it had been altered since 2010.
In a written statement to MPs, Mr Heaton-Harris said MI5 had increased the threat level to the region from Northern Ireland-related terrorism independently of ministers.
"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.
He said that despite the success of the Good Friday Agreement "a small number of people remain determined to cause harm to our communities through acts of politically motivated violence".
"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.
"These attacks have no support, as demonstrated by the reaction to the abhorrent attempted murder of DCI Caldwell."
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