Staff at the Royal Hospital neo-natal unit in Belfast are trying to find the source of the bacterial infection linked to the deaths of three babies.
The first child died on 6 January, the second on 13 January - both from the Pseudomonas infection. The third child died on Thursday night.
It emerged on Friday that the infection has affected four other babies.
Two of the babies have recovered, another is being treated, and a fourth died of an unrelated cause.
It has also emerged that in December at Altnagelvin Hospital in Londonderry, a baby died of a different strain of the bacterial infection.
The Department of Health in Northern Ireland said there was no evidence to link the current outbreak in the Royal Hospital with that of the incident at Altnagelvin in December.
It said all necessary control measures were put in place at the time and "the infection was eradicated".
A helpline has been set up for parents on 028 90 635 389.
The chief executive of the Belfast Health Trust said a full investigation was being carried out.
However, Colm Donaghy said that the "first priority is the safety of the babies and ensuring the work that we do keeps babies safe".
"I have absolute confidence in the measures being taken by our staff," Mr Donaghy added.
Earlier, Dr Richard Wright , associate medical director at the Belfast Health Trust, said that Pseudomonas would occur in all Northern Ireland's hospitals from time to time but had not been detected in the neo-natal unit in the recent past.
He defended the hospital's response to the first child's death.
"There was no evidence of an outbreak so all appropriate action was taken at the time," he said.
"We have no concerns about mums coming to the hospital to deliver their babies."
Consultant neonatologist Clifford Mayes, who works in the unit, said "we first became aware there was a major problem on Monday night".
He said this followed results of tests which confirmed the second child had died from the same infection as the first.
All babies in the neo-natal unit have now been swabbed for the infection.
Three babies are currently still in a large neo-natal intensive care unit which can hold 13.
They will be moved either on Friday night or Saturday morning to smaller rooms at which point the intensive care room will undergo a "deep clean".
Babies moved to units at other hospitals will be isolated straight away and swabbed again for the infection.
It has been confirmed that two women were transferred from the unit to Dublin earlier this week to deliver their babies.
The bacteria can cause infections in the chest, blood, and urine.
The neo-natal unit contains 31 cots for premature babies and sees about 550 admissions per year. Mr Mayes said as the babies were very vulnerable, the unit could see as many as 30 deaths a year.
Earlier, he said he appreciated that this was an "extremely difficult" time for parents of children in the unit.
He said efforts were under way to identify the source of the Pseudomonas infection.
Earlier, a mother of a baby girl in the unit told the BBC's Stephen Nolan show that she was anxiously waiting for the results.
"She's just seven weeks old and was born at just 24 weeks, weighing just one pound seven ounces," she said.
"She has come so far, past illness and the fact that she could get sick again - we really don't want to think about it."
The Northern Ireland health minister said identifying the source was a priority.
Edwin Poots added: "This is a serious incident.
"I have asked the trust to work with the Public Health Agency to ensure all necessary steps are swiftly taken to identify the source of the infection so that we contain it and reduce the risk of spreading."
He added: "We'll be seeking to isolate babies that haven't contracted the bacteria at this point to ensure further children do not contract it.
"So there's a considerable amount of work that's being done."