Dissident republicans in Northern Ireland want to kill members of the security forces in the run up to the centenary of the Easter Rising, a senior police officer has said.
Assistant Chief Constable Stephen Martin was speaking after a bomb exploded under a prison officer's van.
The device partially detonated when he drove over a speed ramp at Hillsborough Drive in Belfast, at 07:10 GMT.
The 52-year-old man, a father-of-three, is in a stable condition in hospital.
It is understood he works in the Prison Service Training Centre at Hydebank College in Belfast.
The van has been removed from the scene for examination and the road has reopened.
ACC Martin, of the Police Service of Northern Ireland (PSNI), said the threat from dissidents is "severe".
"That means an attack is highly possible," he said.
"In recent weeks we have been increasing patrols across Northern Ireland and we intend to do that in the coming weeks up to and through Easter.
"[This year] is the 100th anniversary of the Easter Rising.
"That's a very important date, particularly for people from the nationalist/republican community."
The rising was a short and ill-fated republican rebellion in 1916 against British rule in Ireland.
In spite of its military failure, it is seen by many historians as a significant stepping-stone in the partition of the island and the eventual creation of the Republic of Ireland.
ACC Martin said: "There are people within dissident republican groupings who want to mark the anniversary in an entirely sinister way, who want to kill police officers, prison officers or soldiers."
He appealed for the help of the community to deal with the dissident threat.
Northern Ireland Secretary Theresa Villiers said the attack showed "how lethal the terrorist threat continues to be".
"Thankfully these incidents happen very rarely but that is only because of the outstanding work of the PSNI and their security partners," she added.
Northern Ireland's First Minister Arlene Foster and Deputy First Minister Martin McGuinness said it was a "disgraceful and despicable attack".
"Our thoughts and prayers are with this senior prison officer and his family as he is treated for his injuries in hospital," they said in a joint statement.
"We join all right-thinking people in condemning these cowardly actions.
"As a prison officer, he is someone who serves and protects our community and we are united in our rejection of this attack."
Finlay Spratt, from the Prison Officers Association, said officers had continued to be targeted since the ceasefires in Northern Ireland's Troubles and the signing of the Good Friday Agreement in 1998.
"There is no let up for prison officers, we can't live a normal life, we're not allowed to live a normal life by these thugs," he said.
"It doesn't matter how often you condemn it, it just seems to go on and on and they're attacking people who are serving all the community."
Three years ago dissident republicans shot and killed prison officer David Black on the M1 motorway as he made his way to work.
Mr Black was the 30th prison worker to be murdered in Northern Ireland since 1974.