"We rely on all the crossings to get our aid in... we want all of those crossings open and we want completely unimpeded access," Fletcher said in an interview in Cairo.

"It should happen now. We want it to happen immediately as part of this agreement," he added, referring to the deal between Israel and Palestinian militant group Hamas, sponsored by US President Donald Trump.

"That's part of the discussions that we've had in Sharm el-Sheikh," Fletcher said, adding that the US president and other world leaders "were unequivocal that we must be allowed to deliver aid at massive scale".

On Monday, Trump and regional leaders signed a declaration in the Egyptian resort to cement the ceasefire deal.

"The test of this agreement is not the photos and the press conferences and the interviews. The test is that we have children fed, that we have anesthetics in the hospitals for people getting treatment, that we have tents over people's heads," Fletcher said.

The war sparked by Hamas's October 7, 2023 attack on Israel led to a humanitarian catastrophe in Gaza, with the densely-populated territory reliant on aid that was heavily restricted by Israel, when not cut off outright.

At the end of August, the United Nations declared a famine in Gaza, though Israel rejected the declaration. 

"We've got 190,000 metric tons on those borders ready to go in," said Fletcher, who on Thursday is to go to the lifeline Rafah crossing between Gaza and Egypt to see preparations there.

The crossing remained shut on Wednesday despite reports that it could reopen to aid convoys, as Israel insisted Hamas hand over the remains of the last deceased hostages it holds.

"I don't know at this stage whether the crossing will open for sure," Fletcher said.

But "we are determined to get in there, stop the starvation, rebuild the health sector, clear the rubble and start to give people hope of a better life", he said, adding: "There's a sense of complete urgency to this." 

Israel's far-right National Security Minister Itamar Ben Gvir has threatened to cut off aid supplies to Gaza if Hamas fails to return the remains of soldiers still held in the territory.

Fletcher said what had aid had entered so far was "a fraction of what's needed", with just "tens of trucks on a good day rather than the hundreds of trucks" required.

"We need shelter for the winter... We need more fuel" for power and sanitation plants, he said.

"I'm hopeful that we will have the conditions to deliver at scale but it's not certain. There is hope right now but it's very precarious hope," he said, noting that a timeline for access "wasn't explicitly set out in the peace agreement that President Trump and other leaders negotiated".

Fletcher noted an appeal for Gaza was only 28 percent funded, calling for urgent support.

"What I heard in Sharm el-Sheikh was a real sense of collective generosity and determination -- but we now need to see the numbers, we need to see that financing comes".