SEOUL (THE KOREA HERALD/ASIA NEWS NETWORK, BERNAMA) - Four babies born prematurely at Seoul's Ewha Womans University Medical Centre baffled hospital staff when they died one after another after suffering cardiac arrest - all within a span of some 80 minutes.

The mystery surrounding their quick successive deaths were not solved on Monday (Dec 18) after initial examinations conducted by forensic doctors failed to pinpoint the causes of their deaths, the Yonhap news agency reported.

The babies were in incubators at the intensive care unit (ICU) for newborns at the hospital in Mokdong, western Seoul, when they died between 9.31pm and 10.53pm on Saturday, police said.

Families said the babies all had bloated bellies and difficulty breathing before their deaths.

"The causes of death cannot be determined through an interim post-mortem examinations," the National Forensic Service (NFS) said in a press release on Monday.

"The cause can only be determined after conducting various tests such as looking at tissue through microscope."

Possible causes of death could include a virus or bacterial infections, malfunctioning incubators or medical malpractice, Yonhap reported.

Officials said it will take about a month before the full autopsy results are finalised.

"As it's been well known, we have a shortage of forensic doctors. But we assigned as many doctors to the case as possible because this is a case shocking to the medical society, and there is a high interest among the people," Dr Yang Kyoung Moo, chief of the forensic medical team at the NFS Seoul Institute, was quoted as saying by Yonhap.

The hospital's public relations director Kim Han Su said at a press conference on Sunday the medical team performed cardiopulmonary resuscitation on the babies following the cardiac arrest, but failed to save all four of them.

Four of the 12 babies remaining at the intensive care nursery after the sudden deaths have been discharged, while eight have been transferred to other hospitals, Mr Kim said.

The Seoul city government, Korea Centre for Disease Control and Prevention (KCDC), and other health authorities are also conducting examinations on the hospital's ICU for newborns.

Earlier, the KCDC said bacterial infections could have killed the babies, citing results of blood culture tests conducted before they died.

Prime Minister Lee Nak Yon has ordered a thorough and swift investigation to determine the cause.

"A tragic incident that shouldn't have happened has happened. I don't know what to say to the parents of these babies," Mr Lee said during a meeting with senior aides, according to his office.