Roy Toner: "We were on a heightened state of alert"

Chief superintendent Roy Toner said the police contacted the prison authorities as soon as they became aware of what he called "suspicious activity".

"We had no specific intelligence but we were on a heightened state of alert," he said.

"We are keeping a fairly open mind but our initial enquiries are focusing on dissident republican groups."

Meanwhile, Ulster Unionist MP Jeffrey Donaldson said demilitarisation should be postponed until "rogue paramilitaries" were dealt with.

The Lagan Valley MP said those responsible represented a "further threat to the peace process".

"It seems it may have been the work of dissident republicans," he said.

Abandoned

"It also demonstrates why the government needs to maintain a high level of security.

"Any thought they might have had for further reductions in security should now be set aside."

Jeffrey Donaldson: Blamed dissident republicans for the attack

The dissident groups are opposed to the peace process in Northern Ireland.

They have been blamed for a series of bomb attacks in the province in recent weeks.

The police are looking for at least two men seen abandoning the vehicle containing the device.

Witnesses saw the white 4x4 and another vehicle approach the prison entrance and watched as the driver of the jeep got out and into the second vehicle, before it sped away.

The men ran from the Mitsbuishi vehicle, which had been stolen in Dublin earlier this month and fitted with false plates, to a waiting car.

Shortly afterwards, the device inside the vehicle exploded.

Fortunately no staff were injured and that is the most important thing

Finlay Spratt Prison Officers' Association

It is understood prison officers inside the complex were given approximately two minutes' advance warning.

Finlay Spratt of the Prison Officers' Association said the attack was designed to intimidate staff and police clearly had intelligence that it was going to happen.

"They got about a two minute warning and that allowed them time to evacuate the members of staff who were at that location," he said.

"Fortunately no staff were injured and that is the most important thing."

An army bomb disposal team spent several hours examining the scene before taking the jeep away for further forensic examination.

The attack comes less than two weeks after a device detonated outside the police training centre at Garnerville in east Belfast.

Meanwhile, there is a security alert on the railway line between Lurgan and Moira.

The line has been closed after a suspicious object was left near Lake St in Lurgan.