Police Service of Northern Ireland (PSNI) Ass Ch Con Stephen Martin said: “The threat is severe. That means an attack is highly possible.
“But we are deeply concerned by the current threat picture. 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.
“Easter is a very very important part of the annual calendar. This year it has much greater and increased significance.
“It’s the 100th anniversary of the Easter Rising. That’s a very important date, particularly for people from the nationalist/republican community.”
“We will be culturally sensitive in our policing of the Easter 16 events,” Ass Ch Con Martin said.
“However, there are people within dissident republican groupings who want to mark the Easter 2016 100th 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 told BBC’s The Stephen Nolan Show that the attack was “a demonstration of how lethal the terrorist threat continues to be in Northern Ireland”.
“Thankfully these incidents happen very rarely but that is only because of the outstanding work of the Police Service of Northern Ireland (PSNI) and their security partners in preventing these attacks from happening most of the time,” she added.
Northern Ireland 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.
“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 prison officers had continued to be targeted since the ceasefires 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 the community, all the community.”
A number of residents have been moved from their homes at Hillsborough Drive and an emergency evacuation centre has been opened at the Salvation Army on the Cregagh Road.