Two premature babies being cared for at the Al-Shifa hospital in Gaza died before the evacuation of 31 others, the UN said, adding that most of those who reached Egypt were "unaccompanied".
The United Nations health agency helped evacuate 31 premature babies from the Al-Shifa hospital in Gaza City on Sunday, lamenting that the hospital had become "a death zone".
Twenty-eight of those babies arrived in Egypt on Monday.
The World Health Organization warned that they were all "fighting serious infections and continue needing health care".
The other three babies moved from Al-Shifa in the north of the Gaza Strip are now in a hospital in southern Gaza and have their families with them.
WHO spokesman Christian Lindmeier told reporters in Geneva on Tuesday that the Al-Shifa hospital had been caring for 33 premature babies just the evening before the evacuation.
"Two of these premature infants died only on that night because of the lack of care available to them," he said.
The UN children's agency UNICEF meanwhile said that 20 of the 28 babies evacuated to Egypt had been "unaccompanied".
"Seven mothers accompanied eight babies," spokesman James Elder told journalists in Geneva, adding that the babies included a pair of twins.
The reasons most of the babies were unaccompanied varied, he said.
Several parents had been unable to cross into Egypt, while others were "orphans".