The third phase of a giant polio vaccination drive targeting children in Gaza began Tuesday in a particularly war-ravaged zone but the WHO said a support convoy had to abort its mission.

After the first confirmed polio case in 25 years, a massive vaccination effort began last week targeting over 640,000 children under 10, aided by localised "humanitarian pauses" in fighting.

After covering central and southern Gaza, the campaign moved into its final phase in the north on Tuesday.

The campaign in the north "will be held from 10 to 12 September, " World Health Organization spokesman Tarik Jasarevic said.

Maher Shamiya, the deputy health minister in the Gaza Strip, told AFP 230 teams were working to provide the vaccines and that there had already been "significant turnout of families eager to vaccinate their children".

"I came to protect my children from polio," said Samah Yahya, a 38-year-old mother of two from Gaza City.

"I heard that the vaccine is safe, and thank God, the children received it."

Disease has spread with Gaza lying in ruins and the majority of its 2.4 million residents forced to flee their homes due to Israel's military assault -- often taking refuge in cramped and unsanitary conditions.

A fresh campaign to provide a needed second dose is due to begin in about four weeks in Gaza, besieged for over 11 months.

Ahead of the roll-out of the third phase, "vaccines, cold chain equipment and finger markers were delivered to north Gaza yesterday," Jasarevic said.

However, "a WHO mission carrying fuel for hospitals and vehicles for the polio campaign as well as campaign monitoring experts was impeded", he said.

It had taken three hours for that mission to get a green light from the Israelis to move, "followed by five hours at the holding point, after which the mission had to be aborted", he explained.

WHO also voiced concern that some areas in the north facing Israeli evacuation orders are part of the areas where humanitarian pauses have been agreed to allow the vaccination to go ahead.

"We appeal to all parties to continue ensuring these humanitarian pause zones are respected during the campaign," Jasarevic said.

Hamas's October 7 attack on Israel that sparked the war in Gaza resulted in the deaths of 1,205 people, mostly civilians, including some hostages killed in captivity, official Israeli figures show.

Israel's retaliatory offensive in the Gaza Strip has so far killed at least 41,020 people, according to the health ministry in the Hamas-run territory.