An expected Israeli assault on Rafah has aid groups scrambling for ways to help the 1.5 million civilians sheltering in the south Gaza city but the uncertain timeline poses a logistical nightmare.
"We always are prepared with plans to upscale or downscale but, really, we don't know what to expect," said Bushra Khalidi, head of advocacy at Oxfam.
Oxfam joined 12 other aid groups in a joint call for a ceasefire on April 3, stressing that more than a million civilians, including at least 610,000 children, were "in direct line of fire" in Rafah.
Prime Minister Benjamin Netanyahu has said repeatedly that Israel will press ahead with the threatened assault on Rafah, the last major population center in Gaza that Israeli ground troops have yet to enter.
The hawkish premier has said that the destruction of the remaining Hamas battalions in Rafah is vital to his government's war aim of destroying the Islamist group in Gaza.
But Israel's staunchest ally the United States has said repeatedly that it opposes any operation in Rafah without credible measures to protect civilians.
The Israeli government said it was planning different evacuation scenarios, including the creation of what military spokesman Daniel Hagari dubbed "humanitarian islands".
He said these tent cities would be spared the fighting and would be set up with international support.
Citing Egyptian officials briefed on the Israeli plan, the Wall Street Journal reported that the evacuation operation would last two to three weeks and be carried out in coordination with the United States and Arab countries including the United Arab Emirates as well as Egypt.
The newspaper said Israel planned to send its troops into Rafah gradually, targeting areas where Israel believes Hamas leaders and fighters are hiding, and expected the fighting to last at least six weeks.
But the aid groups AFP spoke with said they had not been briefed on Israel's plans, and the Israeli army was not able to answer AFP questions on its exchanges with humanitarian organisations.
Earlier this month Israeli media reported that the defence ministry had bought 10,000 tents to be set up outside Rafah over the next two weeks, and planned to acquire 30,000 more.
"I have no idea what the plan with the procurement of tents by the Israelis is," said the head of the UN humanitarian office in the occupied Palestinian territories, Andrea de Domenico,.
Rafah lies hard by the Egyptian border and hosts the main crossing through which aid enters the whole territory.
"We don't exactly know what shape this operation will take, but what is certain is that there will be a decrease of available aid, and that many people are moving around," said Jean-Raphael Poitou, Middle East Director for Action Against Hunger.
Palestinians have limited options in the event of an Israeli ground assault on Rafah.
They could topple the wall and barbed wire separating the city from Egypt, attempt to return to the north of the Gaza Strip, which the army does not currently allow, or flee towards the Mediterranean coast.
"Rafah is tiny, it's like a village, any operation in such a limited and densely populated area, we can only imagine that it would cause mass carnage and further atrocity crimes," Oxfam's Khalidi said.
The aid group fears it will have to put its activities on hold in Rafah, where half of its offices and buildings housing staff are located.
Khalidi said it was difficult to anticipate where services could be moved to when about 60 percent of buildings in the Gaza Strip have been destroyed or damaged, and the Palestinian territory is still subject to air strikes and littered with unexploded ordnance (UXOs).
Other organisations fear for the aid distribution network, a subject of tension with Israeli authorities since the beginning of the war.
An operation "means cutting the aid system off from its lifeline, the Rafah crossing," said Ahmed Bayram, media adviser for the Norwegian Refugee Council in the Middle East.
A UN worker in Jerusalem, who asked not to be identified, told AFP that an Israeli ground operation in Rafah would mean "stepping into the abyss".
"If the current conditions do not allow humanitarian operations at the required scale, just imagine what ground fighting in Rafah will bring about," the UN worker said.
De Domenico warned that humanitarian principles prevented UN agencies from getting involved in establishing replacement displaced persons' camps outside Rafah.
"We will not pre-empt movement or attract movement of people by installing camps," the OCHA official said, adding there was in any case "not much space to do so".
The UN worker in Jerusalem said: "The UN does not participate in forced, non-voluntary displacement".
