Hospitals in the southern Gaza Strip have only three days of fuel left due to closed border crossings, the head of the World Health Organization said Wednesday.

Despite international objections, Israel sent tanks into the overcrowded southern city of Rafah on Tuesday and seized the nearby crossing into Egypt that is the main conduit for aid into the besieged Palestinian territory.

WHO director-general Tedros Adhanom Ghebreyesus said fuel that the UN health agency had expected to be allowed in on Wednesday had been blocked.

The Israeli authorities control the flow of humanitarian aid into Gaza.

"The closure of the border crossing continues to prevent the UN from bringing fuel. Without fuel all humanitarian operations will stop. Border closures are also impeding delivery of humanitarian aid into Gaza," Tedros said on X, formerly Twitter.

"Hospitals in the south of Gaza only have three days of fuel left, which means services may soon come to a halt."

Rik Peeperkorn, the WHO representative in the Palestinian territories, told a press conference that fuel was critical to aid operations.

It is mainly used to power the generators which provide hospitals with the electricity they need to operate, but is also used so humanitarians can move around, and to keep bakeries running.

"What we all need, we humanitarians, is fuel, fuel, fuel," Peeperkorn said.

"Without fuel, all humanitarian operations, including hospital operations -- they come to a halt."

Israel bombarded Rafah on Wednesday as talks resumed in Cairo aimed at agreeing the terms of a truce in the seven-month war.

  'Total blockade' 

 Tedros said Al-Najjar, one of the three hospitals in Rafah, had been forced to shut down due to the ongoing hostilities in the vicinity and the military operation in Rafah.

Its patients have been moved elsewhere and hospital staff were removing supplies and equipment to safeguard them.

"At a time when fragile humanitarian operations urgently require expansion, the Rafah military operation is further limiting our ability to reach thousands of people who have been living in dire conditions without adequate food, sanitation, health services and security," Tedros said.

"This must stop now."

WHO emergencies director Michael Ryan insisted Israel's Rafah incursion could not be characterised as a limited military operation, if "the first act of that offensive is to cut off the two lifelines to 2.5 million people in Gaza", he said, referring to the closed border crossings in the south.

"To stop the fuel, stop the food, stop the medicine at source at the border... I don't call that restricted. I call that a re-imposition of a total blockade."

The WHO has pre-positioned some supplies in warehouses and hospitals, Tedros said, but without more aid flowing into Gaza, it would not be able to sustain life-saving support to hospitals.

Tedros also said that the Nasser Medical Complex in Khan Younis had been cleaned up following an attack and siege earlier this year.

"They have recruited health workers and the hospital is ready to start receiving dialysis patients today," he told the press conference.

Gaza's bloodiest-ever war began following Hamas's unprecedented October 7 attack on Israel that resulted in the deaths of more than 1,170 people, mostly civilians, according to an AFP tally of Israeli official figures.

Vowing to destroy Hamas, Israel has conducted a retaliatory offensive that has killed more than 34,800 people in Gaza, mostly women and children, according to the Hamas-run territory's health ministry.