A 50m exclusion zone was set up around the MV Arklow Meadow as police and emergency crews dealt with the major incident on Wednesday morning.

It is believed aluminium phosphyde gas leaked out after its cargo of the became wet and therefore unstable.

Eighteen people - including 13 crew members and five harbour workers - are suffering the effects of exposure to the gas following the leak, which is being treated as an industrial accident.

Nine were taken to hospital, while another nine were treated at the scene.

It's designed to kill rats and mice so its intention is to kill living organisms so human beings are at exposed to that there is an immediate and a very sustained threat to life.

John Allen, NIFRS

Aluminium phosphyde is a pesticide used to kill small mammals such as moles and rodents.

"This gas is toxic but you must be close to the source of the gas so people who are at a long distance, more than 100m, we were able to establish that they're at no risk whatsoever," said John Allen, Area Commander with the Fire Service.

"It's only people who were in very immediate vicinity, 5-10m metres away, those are the people about whom there's a concern."

In a statement, the Warrenpoint Harbour Authority said an investigation is underway.

A spokeswoman said: "A cargo vessel containing grain was being discharged as normal. Staff noticed that several small packages of pesticide contained within the cargo had started to vaporise.

"We understand that the cargo of grain itself is not contaminated. The crew and staff were immediately evacuated from the ship and the Port's emergency plan was enacted.

"Thirteen crew members and five Warrenpoint Harbour staff came into direct contact with the pesticide fume."

Residents in Newry Street were evacuated for a time to the town hall, but have now been allowed to return home, however the cordon at the harbour remains in place.

Police warned people living nearby to keep their windows and doors closed, and Garda were informed of the potential of the gas to drift into their area.

Fifteen ambulances went to treat people affected by the gas while eight fire engines and three specialist appliances were sent by the fire service.

A fire service spokesperson said they were working with the Public Health Agency (PHA) to find those who came into contact with the gas.

"The PHA would emphasise that there is no risk to the wider public, but asks that people in the general area cooperate with the authorities to ensure that this incident is handled safely and effectively.

"The PHA continues to work with colleagues in the emergency services and Southern Health and Social Care Trust on this incident.

A freephone number for relatives and friends to ask about patients of the incident has been set up on 0800 1114 021.