Staff are to carry out a deep clean at a hospital's neonatal room after an infection killed three babies.
Health officials said four other babies were found to have the infection, caused by a bacteria called pseudomonas, in the wake of the three deaths at the Royal Maternity Hospital in Belfast.
One is currently undergoing treatment, two have been treated and have already recovered, while the fourth made a recovery from pseudomonas but subsequently died of unrelated causes.
A spokeswoman for the Department of Health, Social Services and Public Safety (DHSSPS) said the baby still having treatment was doing well.
A different - and apparently unlinked - strain of the infection also claimed the life of another baby in Altnagelvin hospital in Londonderry last month, health officials said.
Other vulnerable babies have been swabbed for signs of the infection and a helpline has been set up for worried relatives as staff try and trace the source of the infection.
The neonatal room at the Royal is being emptied after the outbreak and will undergo a deep clean over the weekend weekend.
The large intensive care room which can take up to 13 babies was being cleared yesterday and the infants are being separated into small rooms ahead of the cleaning operation.
In regard to the death in Derry in December, officials said the strain of the infection, which was subsequently eradicated within the hospital, was different from the one that has hit the Royal's maternity unit and there was no evidence that the outbreaks were linked.
The bacteria can cause infections in the chest, blood and urinary tract.
Stormont Health Minister Edwin Poots has received regular briefings from health officials as the situation developed.
"It is important that the parents and families of the babies affected are given all the support they require," he said.
"I would also ask people to remain calm. The infection control team is now in the process of trying to identify the source of the infection and our health and social care system is in a good position to make sure all babies receive the neonatal care they require.
"I spent some time today with the doctors and nurses who work in the unit and I want to personally thank them again for all their hard work and dedication during this difficult period."
He stressed that the neonatal unit is the only part of the hospital affected by the pseudomonas outbreak and the delivery wards and all other services at the Royal are operating as normal.
Expectant mothers should attend their appointments as scheduled, he added.
Pseudomonas is not itself infectious but because it exists in water or moisture patients can carry it on their skin. It can be treated with the right antibiotic but the third baby died despite the treatment.
The unit is populated by extremely premature and small infants.
Belfast Health and Social Care Trust chief executive Colm Donaghy said they would be carrying out a full investigation into whether anything else could have been done.
The first death happened on January 6 and the second on January 13. The third infant died late last night.
Doctors first became aware that there was a problem with the infection on Monday night when the laboratories reported it.
Some babies may be moved to other health trusts or even outside Northern Ireland if the pressure becomes too great.
It will take at least a week to find out if there were deaths relating to different strains of the infection. Usually there are fewer than 80 cases of it annually across Northern Ireland.
Two heavily pregnant women have been forced to travel 100 miles to Dublin to have their babies because of the outbreak.