The officers were from the south Co Down area and died when their car crashed on a stretch of coastal road between Kilkeel and Warrenpoint at 4am.

Chief Constable Sir Hugh Orde led tributes to the men who were aged in their mid-20s to mid-30s, while it is understood two had young children.

There were no other vehicles involved in the crash and a senior officer will now investigate the tragedy, including claims a member of the public made a desperate bid to rescue the men.

The two full-time and two part-time officers have yet to be identified, but one who was named locally as Declan Greene lost three members of his family in an infamous Co Down fishing tragedy six years ago.

Sir Hugh said: "This is an extremely sad day for the family, friends and colleagues who served with the police officers who lost their lives in this tragic incident.

"The police family have lost four fine police officers in what are deeply tragic circumstances and our heartfelt condolences go out to their families.

A "black box" recorder in the police Shogun vehicle will be key to uncovering what happened and police said they will investigate unconfirmed reports that the blaze set off ammunition in the car.

Police said the crash was still being investigated, but last night ruled out speculation the men had been trapped in the vehicle by a locking system and said: "We can clarify that the Shogun vehicle involved in the incident does not use self-locking or automatically-locking doors."

In 2002 three members of the Greene family, eight-year-old Michael and his father and grandfather who were also called Michael, died when their fishing vessel, the Tullaghmurry Lass, sank off the Co Down coast.

The tragedy devastated the community and last night a priest from the area, Fr Michael Murray, said Mr Greene was a nephew of the eldest of the three family members to die in the fishing tragedy.

Fr Murray said Mr Greene, who was married with children, was widely respected.

"He was a total gentleman and always willing to help people in whatever way he could," he said.

"There were prayers for the family at Mass this morning. After the earlier tragedy, this is a devastating blow."

Assistant Chief Constable Judith Gillespie, responsible for the Police Service of Northern Ireland's (PSNI) rural region, said the men were on their way to help a fellow officer when their vehicle hit a wall at the Moygannon bridge and caught fire.

"Today we mourn the deaths of four police officers, who lost their lives as they supported their colleagues in serving their community," she said.

"We feel this loss keenly and our thoughts are with the families and friends of those police officers."

The crash is the largest loss of life for the PSNI since the paramilitary ceasefires of more than a decade ago.

Ms Gillespie said messages of condolence had been sent from across the UK and Ireland.

Yesterday's tragic deaths came after three other people were killed in separate road accidents on Friday night.

First Minister Peter Robinson and deputy First Minister Martin McGuinness expressed their deep sorrow.

Mr Robinson said: "I am saddened at the loss of life on our roads this weekend. Seven people have been killed including four police officers who died as the result of a tragic accident while serving their community. My thoughts and prayers are with all the victims' families and loved ones."

Mr McGuinness said: "This has been a tragic weekend on our roads. I would like to pass on my condolences to all the victims' families at this very difficult time."