The neo-natal unit at the Royal Maternity Hospital in Belfast has been temporarily closed to new admissions after three premature babies picked up a life-threatening infection.
Specialists are battling an enterobacter infection in the special and intensive care units at Northern Ireland's main facility for vulnerable babies.
The hospital spokeswoman said: "The facility has been closed to new admissions and we don't know for how long. They are taking specialist advice from the infection control team."
The babies have been separated from healthy infants and are being treated with strong antibiotics.
There are 21 infants in the maternity unit who have not contracted the bacteria. Consultant neonatologist Dr David Sweet said expectant mothers should not be too alarmed.
"I think people should be reassured that we have acted quickly to put a lid on it. We have asked our colleagues in other hospitals to help us and where necessary we are going to try and make sure that they have intensive care unit facilities available."
Dr Sweet added that the outbreak was not highly contagious but said he was concerned because the babies were vulnerable, with immature immune systems.
He said that cases of enterobactor infection were not common but did affect most big intensive care units from time to time. He added that units in other hospitals would be reluctant to admit babies from the Royal until the threat of infection had been cleared.