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A group calling itself the New IRA has claimed responsibility for a Semtex bomb attack that injured a prison officer in Belfast, according to reports.
The 52-year-old, a married father of three, required surgery after an explosive device detonated under the van he was driving on Friday morning. His condition has been described as stable.
Four people including a woman have since been arrested.
In a statement to the BBC, the dissident republican group reportedly said the officer was targeted because he was involved in training other guards at HMP Maghaberry, near Lisburn.
A spokesman said the officer was one of a number on a list of potential targets and the attack arose from a dispute over the treatment of dissident Republican inmates.
The group claimed to have used the plastic explosive Semtex and a commercial detonator in the attack.
Police investigating the attempted murder of a prison officer in Belfast on 4 March have arrested three men and a woman, the Police Service of Northern Ireland has said.
There are fears of a new terror campaign in Northern Ireland after a prisoner officer was seriously injured when a bomb exploded under his van.
The incident was a flashback to a troubled past and a warning, say police, about the immediate future.
Police warned there could be more attacks marking the 100th anniversary of the Easter Rising.
ITV News Correspondent Neil Connery reports from Belfast:
The terror threat level in Northern Ireland has been upgraded to "the upper end of severe," Police Service of Northern Ireland has said today.
The warning follows a car bomb attack in the east of the city this morning in which a prison officer was injured. PSNI Assistant Chief Constable Stephen Martin said that the attack had "set out to kill."
He expressed fear that the attack was part of an upsurge in dissident activity ahead of the forthcoming Easter Rising anniversary and that dissident republicans are intent on killing security force members to mark the 100th anniversary of the republican rebellion against British rule on the streets of Dublin.
The 52-year-old long-serving prison officer, who is married with children, required surgery after an explosive device detonated under the van he was driving on Friday morning. His condition has been described at stable.
The terror threat level in Northern Ireland has been categorised as "severe" for a number of years.
The PSNI has released a video appealing for people to come forward with information relating to this morning's car bomb attack in Belfast, telling the public that those who planted the device under a prison officer's van had "set out to kill".
The unnamed 52-year-old victim of the attack is currently undergoing treatment in hospital after the bomb exploded as he drove to work at around 7:10am in the Woodstock Road area.
"'There is a severe threat in Northern Ireland," Assistant Chief Constable Stephen Martin says in the video. "That means an attack is very likely and we're very worried about the threat picture."
He acknowledged the upcoming 100th anniversary of the Easter Rising and that some dissident republicans may want to mark the centenary "in an entirely more sinister way."
Deputy First Minister Martin McGuinness has tweeted his condemnation of this morning's incident in east Belfast.
A 52-year-old prison officer was on his way to work when after an explosive device detonated under the van he was driving in the Woodstock Road area.
He is in a stable condition in hospital.
A shop worker waiting to gain permission to open up in the Woodstock Road area of Belfast said he had heard a "massive" bang at about 7.20am.
"I didn't know what it was but it was huge," said the man who declined to be named.
A large cordon has been put up around the scene with several streets closed off. Part of a twisted bumper can be seen lying at the side of the road.
A prison officer injured in the explosion is in hospital. It is understood the man's injuries are not considered life threatening.
Stormont First Minister Arlene Foster has condemned the incident in Belfast this morning:
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The extent of the man's injuries is not yet known.