A third baby has died after an outbreak of an infection at the Royal Maternity Hospital in Belfast.
A bacteria called pseudomonas is suspected to have caused the deaths and other vulnerable babies have been swabbed for signs of it.
The bacteria can cause infections in the chest, blood and urinary tract.
Health officials in Northern Ireland also revealed that a different - and apparently unlinked - strain of the infection claimed the life of another baby in Altnagelvin hospital in Co Derry last month.
Belfast Health and Social Care Trust Chief Executive Colm Donaghy said the first death happened on 6 January and the second on 13 January. The third infant died late last night.
A neonatal room at the hospital is being emptied and the area where they were being treated will undergo deep-cleaning after the remaining three patients have been moved.
Admissions have been restricted to all but the most seriously ill babies who cannot be treated elsewhere, but staff believe the infection has not spread to other areas of the Royal Victoria Hospital site.
Investigations are continuing to establish the cause of the outbreak at the unit where 26 other babies are being cared for.
Northern Ireland Health Minister Edwin Poots said expectant mothers are likely to be transferred to other hospitals in the Republic and Britain.
Pseudomonas aeruginosa is a germ or bacteria found in soil, water, plants and animals.
It 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.
Mr Donaghy said it is a germ which can be on your skin and not cause any harm but with a sick patient it can cause very serious problems.
Parents entering the neonatal ward are being asked to be careful about hand hygiene to prevent the spread of the infection.
Dr Richard Wright, associate medical director at the Belfast Health and Social Care Trust said: "We are engaged in a fairly major investigation to identify the cause of the infection."
