The Israeli military said on Wednesday that it has reopened its Kerem Shalom crossing into Gaza after days of closure, but the UN said no humanitarian aid has yet entered and there is no one to receive it on the Palestinian side after workers fled during Israel's military incursion in the area.

The Kerem Shalom crossing between Gaza and Israel was closed over the weekend after a Hamas rocket attack killed four Israeli soldiers nearby, and on Tuesday, an Israeli tank brigade seized the nearby Rafah crossing between Gaza and Egypt, forcing its closure. The two facilities are the main terminals for entry of food, medicine and other supplies essential for the survival of Gaza's population of 2.3 million Palestinians.

The Israeli foray did not appear to be the start of the full-scale invasion of the city of Rafah that Israel has repeatedly promised.

But aid officials warn that the prolonged closure of the two crossings could cause the collapse of aid operations, worsening the humanitarian crisis in Gaza, where the UN says a “full-blown famine” is already underway in the north.

The United States paused a shipment of bombs to Israel last week over concerns that Israel was approaching a decision on launching a full-scale assault on Rafah, in a further widening of divisions between the two close allies.

The US says it is concerned over the fate of around 1.3 million Palestinians crammed into Rafah, most of whom fled fighting elsewhere. Israel says Rafah is Hamas' last stronghold and that a wider offensive there is needed to dismantle the group's military and governing capabilities.

The US, Egypt and Qatar are meanwhile ramping up efforts to close the gaps in a possible agreement for at least a temporary cease-fire and the release of some of the scores of Israeli hostages still held by Hamas.

Israel has linked the threatened Rafah operation to the fate of those negotiations. CIA chief William Burns, who has been shuttling around the region for talks on the cease-fire deal, met Wednesday with Israeli Prime Minister Benjamin Netanyahu, a US official said, speaking on condition of anonymity to discuss closed-door negotiations.

With the seizure of Rafah, Israel now controls all of Gaza's crossings for the first time since it withdrew troops and settlers from the territory nearly two decades ago, though it has maintained a blockade with Egypt's cooperation for most of that time.

The Rafah crossing has been a vital conduit for humanitarian aid since the start of the war and is the only place where people can enter and exit. Kerem Shalom is Gaza's main cargo terminal.

UN World Food Program deputy executive director Carl Skau told The Associated Press that the UN, agency has lost access to its Gaza food warehouse in Rafah, which he says was “communicated as a no-go zone."

“We understand that it's still there, but we are extremely worried of looting,” Skau said during a visit to neighboring Lebanon, adding that a UN logistics warehouse in Rafah had already been looted. He said the WFP was able to secure a warehouse in Deir al-Balah, in central Gaza, but has not stocked it with food yet.

Associated Press journalists heard sporadic explosions and gunfire in the area of the Rafah crossing overnight, including two large blasts early Wednesday. The Israeli military reported six launches from Rafah toward the Kerem Shalom crossing on Tuesday.

COGAT, the Israeli military body in charge of Palestinian civilian affairs, said the Kerem Shalom crossing reopened early Wednesday. But Juliette Touma, the director of communications for UNRWA, said no aid had entered as of late afternoon on Wednesday and that the UN agency had been forced to ration fuel, which is imported through Rafah.

Gaza's Health Ministry meanwhile said at least 46 patients and wounded people who had been scheduled to leave Tuesday for medical treatment have been left stranded.

UN agencies and aid groups have ramped up humanitarian assistance in recent weeks as Israel has lifted some restrictions and opened an additional crossing in the north under pressure from the United States, its closest ally.

But aid workers say the closure of Rafah, which is the only gateway for the entry of fuel for trucks and generators, could have severe repercussions, and the UN says northern Gaza is already in a state of “full-blown famine.”

Skau of the WFP said some food has been delivered to the north in recent weeks.

“When we got up there, people were coming out of the rubble extremely weak, not even able to carry the box of food," he said, adding that an increase in infectious diseases among children could worsen the crisis in the north.

“It's that combination of widespread disease and acute malnutrition that is that deadly cocktail,” he said.

COGAT said 60 aid trucks entered through the northern crossing on Tuesday. Some 500 trucks entered Gaza every day before the war.

The war began when Hamas militants breached Israel's defenses on October 7 and swept through nearby army bases and farming communities, killing some 1,200 people, mostly civilians, and abducting another 250. Hamas is still believed to be holding around 100 hostages and the remains of more than 30 others after most of the rest were released during a November cease-fire.

The war has killed over 34,800 Palestinians, according to Gaza health officials, and has driven some 80 per cent of Gaza's population of 2.3 million Palestinians from their homes. Israel's military campaign has been one of the deadliest and most destructive in recent history, reducing large parts of Gaza to rubble.

Biden has repeatedly warned Netanyahu against launching an invasion of Rafah. But Netanyahu's far-right coalition partners have threatened to bring down his government if he calls off an offensive or makes too many concessions in the cease-fire talks.

The US has historically provided Israel enormous amounts of military aid, which has only accelerated since the start of the war.

The paused shipment was supposed to consist of 1,800 2,000-pound (900-kilogram) bombs and 1,700 smaller ones, with the US concern focused on how the larger bombs could be used in a dense urban setting, a US official said Tuesday on the condition of anonymity to discuss the sensitive matter. The official said no final decision had been made yet on proceeding with the shipment.