Speaking to Reuters, WHO's regional emergencies director Rick Brennan said the Palestinian Red Crescent lacks sufficient fuel for its ambulances within Gaza to evacuate patients.
Egypt was open to having its ambulances cross into Gaza to help evacuate people as long as security guarantees and safe passage could be provided, he said in an interview from Cairo.
The WHO understood that there were still about 600 patients including 27 in critical condition at Al-Shifa hospital, which Israeli forces entered this week following a days-long siege, Brennan said.
"We are looking at the case for full medical evacuation but there are a lot of security concerns, there are a lot of logistics constraints. Our options are rather limited, but we hope to have some better news in the next 24 hours or so," he said.
Those given priority in an evacuation would include the critically ill and 36 newborn babies who lost access to incubators because of lack of fuel to generate power, he said.
Plans for an evacuation had been complicated by the fact that communications with the hospital had been cut most of the time, Brennan said.
"The idea is that we would bring the majority of patients over days or weeks from Shifa," he added.
"We'd bring the bulk of them to hospitals in southern Gaza but those hospitals are already overwhelmed as well so that's another complicating factor. The other option is of course to bring a number of them to Egypt."