The World Health Organization called Thursday for a daily flow of humanitarian aid into the Gaza Strip beyond the first convoy readying near the border -- including fuel to keep hospitals running.

Egypt's state-linked television Al Qahera News reported that the Rafah border crossing between Egypt and Gaza would open Friday, and that Israel had agreed to allow an initial 20 trucks carrying relief goods to pass through.

"We welcome Israel's announcement yesterday that it will not block the entry of water, food and medicines into Gaza from Egypt," WHO chief Tedros Adhanom Ghebreyesus said.

"Fuel is also needed for hospital generators, ambulances and desalination plants -- and we urge Israel to add fuel to the life-saving supplies allowed to enter Gaza," he told journalists.

WHO emergencies director Michael Ryan said Gazan hospitals had actually run out of fuel "days ago".

"The fuel is gone. What's left are tiny amounts of fuel that are being repurposed within the UN and NGO system to desperately try to provide a couple of days' extra fuel, where that can be done," he said.

"We passed the cliff edge long ago."

Tedros said the trucks in Egypt bearing WHO medical supplies were "loaded and ready to go".

"We urge those who can do it to please, please make this happen, to avoid the tragedy in front of us," he said of the need to get aid into the Gaza Strip.

Ryan said that efforts to get aid across the Egyptian border, the only route into or out of Gaza not controlled by Israel, needed to go beyond the "gesture" of the first convoy.

"We need to make sure the corridor is a corridor. Humanitarian assistance needs to move every day. Twenty trucks is a drop in the ocean of need right now in Gaza," he said.

"It shouldn't be 20 trucks: it should be 2,000 trucks.

"Hopefully this trickle will turn into a river of aid that will flow in the coming days."