More than 400 homes have been evacuated in Newtownards, Co Down as authorities attempt to mitigate a "suspected historic piece of munition" that is believed to be a WWII bomb.

"The sheer size of the World War II bomb states that we have to put in place a 400m cordon," PSNI Superintendent Johnston McDowell said on Monday while speaking to the press about the ongoing operation.

"Anyone within that, without a mitigation, there is a serious risk of death or serious injury."

The Police Service of Northern Ireland (PSNI) said in an update on Monday that it had commenced an evacuation operation following the discovery of "a suspected historic piece of munition" on Thursday morning, August 15.

“In excess of 400 homes have been affected by this operation which is a significant number of properties, and partners and I would like to thank residents for their patience and understanding as the operation continues," Superintendent McDowell said in a statement on Monday.

“If homeowners or residents have not been spoken to directly by police, or have not received information in relation to this matter, it’s unlikely that they will be impacted by the clearance operation at this time."

Superintendent McDowell continued: “The operation has now moved into the mitigation phase and work has commenced on establishing protection to nearby properties and facilities.

“There is no definite timescale, however previous operations of this nature have shown that this type of operation commonly takes five days or more before the area is safe to enter.

“A police cordon remains in place and will do for the duration of the operation, with road users being advised to continue to avoid the area.

“Whilst I appreciate the frustration and disruption that this has caused, keeping people safe is paramount and we will not take any risks.

“If you have been affected by this incident, please read the FAQ document in this link: PSNI.Police.uk/rivenwood."

According to BBC News NI, the PSNI issued a leaflet to residents in Rivenwood on Saturday evening asking them to leave their homes by 10 am local time on Sunday due to the "possibility of risk to life and property."

As the PSNI operation continues, Ards Blair Mayne Wellbeing and Leisure Complex has been opened as an emergency support center where displaced residents who have no alternative accommodation can go for shelter and advice.

While the Sports Hall will be closed for all activities until further notice, the Dome was set to reopen on Monday evening for current bookings and will be available as normal for classes and activities including junior activities.

BBC News NI reported on Monday that James Fraser from the developer Fraser Partners said a groundwork contractor was digging a service trench about 1.5m deep when he noticed "something that didn't look right."

He said the contractor spoke to a supervisor who was able to identify the device as a wartime bomb and contacted the PSNI.

"What we're looking at here is a legacy of the Second World War," Dr. James O'Neill of the Northern Ireland War Museum told UTV on Monday. 

"In all likelihood, this is a German bomb and it came here on the night of 15 / 16 of April 1941, during what's known as the Belfast Blitz, but in reality, there were other bombs dropped in Bangor, Derry, and, in this case, Newtownards."

Dr. O'Neill said that Newtownards was a military airfield at the time but that it wasn't a target during the 1941 blitz.

Separately, military historian Jonny McNee told BBC News NI that he also believes the bomb probably came from "one of the raids that the Germans carried out on the Newtownards airfield on the 15 and 16th April 1941.''

He said the Luftwaffe used two bombs - ''250kg and 500kg ones'' - and a 500kg bomb is thought to be the one found in Rivenwood.

''They were very dangerous then, and they are extremely dangerous now, as the intervening years have made them much more unstable, particularly the fuses and the explosives in them.

''The bomb has been lying in the ground for 80 odd years, decomposing, rotting away, the fuses become unstable, and it doesn’t take much to bring those electric clockwork fuses back into life.

''What was dropped by the Luftwaffe in this raid is one of the standard high explosive general-purpose bombs that could have taken out several houses.''