Northern Ireland First Minister Arlene Foster says she was taken by surprise by the increased threat of a terror attack in Britain by dissident republicans.
MI5 said there is a “strong possibility” that republicans opposed to the peace process will mount an attack outside Ireland.
In its latest security assessment, the threat to Britain from paramilitary factions was increased from moderate to substantial - the third most serious category out of five.
In Northern Ireland, the threat level remains at severe, meaning an attack is "highly likely".
Ms Foster said the development was alarming and unexpected.
“We were rather surprised to hear the threat had increased - albeit not to severe, but to substantial, in terms of Great Britain,” she said.
"To me, that means we have to work even harder in terms of the administration at Stormont and to work very hard in relation to our paramilitary strategy, the resources we have put in to deal with paramilitarism and criminality and indeed to work with our neighbours in the Republic of Ireland as well to make sure we eradicate this threat for once and for all."
Dissident republican groups such as the New IRA and Oglaigh na hEireann have been behind several deadly attacks on security forces in recent years.
They claim to have up to a tonne of “newly acquired” Semtex - odourless plastic explosives used widely in a bombing campaign by the Provisional IRA during the 1980s.
Fears are also mounting about their access to sophisticated weaponry in recent years like mortar bombs and high-calibre assault rifles.
Ms Foster said the murder in March of prison officer Adrian Ismay, killed by an under-car bomb as he drove to work, showed the capability of the breakaway factions.
"It is alarming to hear that the threat level has increased in Great Britain and obviously we will want to keep an eye on that and monitor the situation and work with the security services in the UK and also the Republic of Ireland," she told the BBC's Andrew Marr Show.