A prematurely-born infant died after infecting four other babies at a Swedish hospital with bacteria resistant to antibiotics, doctors said on Friday.

Doctors suspect the four babies, all under four months of age, caught an extended-spectrum beta-lactamase (ESBL) strain of bacteria after the now-deceased child was admitted to a hospital in the western city of Gothenburg in early July, daily newspaper Dagens Nyheter reported.

"We are now trying to find out where the disease has come from. We do not know how it emerged from the start," Bjorn Andersson, head physician at Sahlgrenska University Hospital, told the newspaper.

He ruled out that the child had died from the bacteria but said ESBL posed a problem in infants due to their high risk of infection.

The four babies are being treated in isolation at the hospital as doctors considered administering particular antiobiotics that could help fight the infection.

The hospital was due to review its routines and hygiene procedures to prevent a similar outbreak in the future, Andersson said.