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.