Staff are to carry out a deep clean at a hospital's neonatal room after an infection killed three babies.
Health officials in Northern Ireland 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 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 to 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. The large intensive care room, which can take up to 13 babies, was being cleared 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. 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.
Stormont Health Minister Edwin Poots 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.
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. 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.