United Nations agencies called "on our knees" on Tuesday for aid to be allowed unimpeded into Gaza, saying more than 20 times current deliveries were needed to support its Palestinian population after two weeks of Israeli air strikes.

U.N. organisations have been making increasingly desperate appeals since Israel imposed a full blockade on the coastal strip and began bombardments to root out Hamas who had killed civilians in a bloody raid into southern Israel.

Trucks of aid began moving into Gaza from Egypt on Saturday after intense diplomatic efforts, but the agencies say they are far from enough. Half of the enclave's 2.3 million people are homeless, many people have been wounded and food and clean water is in short supply.

"The aid which resumed from Egypt over the weekend is a mere drop in the ocean of what is needed," said Jeremy Laurence, Spokesperson for the Office of the UN High Commissioner for Human Rights (OHCHR).

Fuel, which has not been sent to the Gaza Strip along with the humanitarian aid, was crucial, the United Nations Relief and Works Agency for Palestine Refugees in the Near East (UNRWA) said.

"Fuel is extremely urgent because without fuel, the trucks themselves cannot move," UNRWA spokesperson Tamara Alrifai said. "Without fuel, the generators cannot produce electricity for hospitals, for bakeries and for the water desalination plant.

Brian Lander, deputy head of emergencies at the World Food Programme, said that some 465 trucks of humanitarian aid were needed per day to support the population in Gaza prior to the conflict.

"We're seeing at best 20 trucks a day at the moment," he said, stressing that the people of Gaza were dire need of water, food and other essential products.

WHO said medicines and health supplies had been delivered to three key referral hospitals in southern Gaza but that it still needed to reach the north of the Palestinian enclave, one of the most densely-populated places in the world.

"We still have not been able to reach the hospitals in the north with the medical supplies or the desperately needed fuel," said Dr Rick Brennan, WHO Regional Emergencies Director for Eastern Mediterranean Region.

Brennan said one-third of hospitals in the Gaza Strip were now non-functional at a time when the medical burden is enormous, and that some two-thirds of clinics are not functioning.

"We are on our knees asking for that sustained, scaled up, protected humanitarian operation," he said.

"We appeal to all of those in a situation to make a decision or to influence decision-makers to give us the humanitarian space to address this human catastrophe."