A murder investigation has begun following the death of prison officer Adrian Ismay in Belfast.

Mr Ismay, 52, died 11 days after a bomb attack during which he sustained leg injuries after a device detonated under his van on 4 May.

Dissident republican group the New IRA claimed responsibility for the attack.

The father-of-three was said to have been recovering well but was taken back into hospital yesterday where he died.

In a statement this afternoon, the PSNI said a post-mortem concluded that Mr Ismay died as a direct result of the injuries sustained during the explosion.

Following the attack on Mr Ismay, the PSNI warned that violent dissidents were trying to escalate their activities to mark the centenary of the 1916 Easter Rising.

Detective Chief Inspector Richard Campbell said: "Today is yet another difficult day for the Ismay family, his friends and colleagues as they struggle to come to terms with the events of the past 12 days.

"This has been treated as a significant serious crime investigation from the outset and will continue as such."

One man has already been charged with attempted murder.

DCI Campbell added: "We are liaising with the Public Prosecution Service in relation to the individual who is currently charged with attempted murder and causing an explosion with intent to endanger life."

Mr Ismay trained other guards at HMP Maghaberry near Lisburn and was based at Hydebank Wood Young Offenders Centre in south Belfast.

The attack happened in the Hillsborough Drive area, off Woodstock Road, a predominantly loyalist area in the east of the city.

Minister for Foreign Affairs Charlie Flanagan this afternoon said he condemns in the strongest possible way attacks on prison officers and security forces in Northern Ireland.

Speaking in Washington, Mr Flanagan said there was no place for such attacks in any democracy.

He offered his condolences to Mr Ismay's family.