Injured Palestinians arrive at Al Shifa Hospital following Israeli airstrikes on Gaza City. AP

Israel is consistently blocking humanitarian convoys into northern Gaza, making it increasingly difficult to bring desperately-needed fuel and other aid to hospitals there, the United Nations said on Friday.

After planning aid missions to the north, UN agencies said their convoys were subjected to slow and unpredictable inspections and then a near-systematic refusal from the Israeli side to proceed.

"Operations in the north (are) increasingly more complicated," Andrea De Domenico, head of the UN aid agency OCHA's office in the occupied Palestinian territories.

A Palestinian child wounded in an Israeli strike receives treatment at Nasser hospital in Khan Younis. Reuters

Speaking from Jerusalem to a virtual press briefing, he described how detailed coordination was required with a network of checkpoints, and "the Israelis have systematically, or quasi-systematically, refused" to let them through. In recent days, he said the agency had had three missions partially approved out of 21 requested.

Lucia Elmi, special representative for the UN children's agency UNICEF in the Palestinian territories, also lamented that "we can't get sufficient aid in."

"The inspection process remains slow and unpredictable, and some of the materials we desperately need remain restricted, with no clear justification," she said.

'INHUMANITY'

De Domenico said the Israeli military was particularly wary about allowing fuel into the north, and especially to hospitals. "They have been very systematic to not allow us to support hospitals, which is something that is reaching a point of a level of inhumanity that for me is beyond comprehension," he said.

The UN's World Health Organisation (WHO) meanwhile said that it had finally on Thursday managed to reach Al Shifa Hospital in the north for the first time in over two weeks, after seven failed attempts.

WHO chief Tedros Adhanom Ghebreyesus said on X, formerly Twitter, that the mission allowed for the delivery of desperately-needed aid, including 9,300 litres of fuel.

He hailed that "the team reported that Al-Shifa, previously Gaza's premier hospital, has (partially) re-established services".

The hospital, which WHO described as "a death zone" after it largely ceased operations following raids and occupation by Israeli troops in November, now has 60 medical staff, Tedros said.

It also has "a surgical and medical ward with 40 beds, an emergency department, four operating theatres, basic emergency obstetric and gynaecologic services." Hospitals, protected under international humanitarian law, have repeatedly been hit by alleged Israeli strikes in Gaza since the war erupted.

The Israeli military accuses Hamas of having tunnels under hospitals and using the medical facilities as command centres, a charge denied by the Islamist group.

'DISASTER OF EPIC PROPORTIONS'

Only 15 of Gaza's 36 hospitals are even partially functioning, most of them in the south. The UN has long described desperate scenes in the few barely functioning hospitals remaining in the north, facing severe shortages of food, clean water, medicines and fuel.

While the partial resumption of services at Al Shifa was good news, Tedros emphasised that it meant "the consumption of fuel is much higher, and the need for medical supplies is increasing."

Elmi meanwhile stressed the urgency of allowing more aid through, especially for Gaza's children. "Children in Gaza are running out of time, while most of the lifesaving humanitarian aid they desperately need remains stranded between insufficient access corridors and protracted layers of inspections," she said. "Mounting needs and a constrained response is a formula for a disaster of epic proportions."