Hamas says it is drawing up lists of the wounded - estimated at about 600
Egypt says the Hamas militant group, which controls Gaza, is preventing hundreds of wounded Palestinians from leaving for treatment in Egypt.
Cairo says dozens of empty ambulances are at the Rafah crossing - the only one to Gaza which avoids Israel.
Hamas said it was drawing up lists of the injured but that it was difficult getting them to the border because of the ongoing Israeli air strikes.
Egypt has summoned Israel's ambassador to demand an end to the bombardment.
Hamas officials say 271 Palestinians have been killed and 600 wounded since Israel began its aerial assault on the Gaza Strip on Saturday, but none of the injured have yet left via Rafah.
Egypt has helicopters and doctors on standby at the Rafah crossing.
There are also up to 40 ambulances waiting to go into Gaza to bring out the most seriously wounded. Tonnes of medical supplies have arrived at the nearby airport of El-Arish.
But the Egyptian authorities say that, at the moment, they have no-one to treat.
Egypt's Foreign Minister Ahmed Aboul Gheit said the wounded were "barred from crossing" and he blamed "those in control of Gaza" for putting the lives of the injured at risk.
Gaza has been crippled by an Israeli blockade of all but the most essential supplies, leaving even basic medicines in desperate shortage.
The UN Relief Works Agency - the largest humanitarian organisation working in Gaza - said it has had to stop distributing food and that hospitals were "near to collapse".
Spokesman Christopher Gunness told the BBC that the organisation was struggling to maintain its 18 clinics in Gaza.
"[The hospitals] were strangled and starved of essential drugs in the last few months so it's inevitable that they are at a point of collapse when they're faced with the influx of patients that they're now facing," he said.
"The guns simply must fall silent and people must start talking peace."