BELFAST (Reuters) - A prison officer was injured by a bomb which exploded under his van in Belfast on Friday, police said, warning of a “severe” threat to security forces as the centenary approaches of the 1916 anti-British Easter Rising.

Police said the bomb went off shortly after the man started driving away from his home. The 52-year-old was undergoing surgery in hospital but his injuries were not life-threatening.

A 1998 peace deal largely ended three decades of violence in Northern Ireland between Protestants who want to remain under British rule and Catholics favouring unification with Ireland, but violence sporadically erupts.

More attacks were “highly likely” in the coming weeks, Assistant Chief Constable Stephen Martin told a news conference.

“This could have been a fatality, the people who planned this wanted to kill a prison officer,” Martin said, blaming the attack on nationalists opposed to British rule.

“There are people within dissident republican groupings who want to mark this centenary by killing prison officers, police officers or soldiers,” he added.

“We believe the threat is extremely high, at the upper end of severe.”

The rebellion 100 years ago, centred in Dublin, paved the way for the Irish republic to achieve independence, while the six counties of Northern Ireland remained in the United Kingdom. It will be marked by events on both sides of the Irish border this month.

Martin said police had been stepping up patrols and would try to be “culturally sensitive” at events to mark the uprising, the most dramatic chapter of Ireland’s independence struggle.

Politicians on both sides of the historical divide condemned Friday’s attack.

Northern Ireland’s First Minister Arlene Foster called it “disgraceful and despicable”. Foster’s father, a police officer, survived being shot by the Irish Republican Army (IRA) in the 1970s during the violent period known as the “troubles”.

Deputy First Minister Martin McGuinness, a former commander in the IRA during the troubles and a senior member of the nationalist Sinn Fein party, also condemned what he called a “despicable and futile act”.

In 2012, a Northern Irish prison officer was killed in a motorway shooting blamed on militant nationalists - the first such killing of a prison officer since 1993.

The victim of Friday’s attack worked in a training school rather than in a prison, Finlay Spratt, head of the Northern Irish Prison Officers Association, told Reuters.

“This is just terrible. What can you say? We have been down this road before. These people have no justification for what they do,” Spratt said.