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.

After the first confirmed polio case in 25 years, a massive vaccination effort began last week targeting at least 90 percent of children under 10, aided by localised "humanitarian pauses" in fighting.

"We are confident that we probably reached the target," Rik Peeperkorn, the WHO's representative for the Palestinian territories, told reporters.

WHO had initially said it aimed to vaccinate some 640,000 children, but Peeperkorn said that had likely been an overestimate of the target population.

By Wednesday, he said, 552,451 children had been reached with a first dose of the vaccine, adding that WHO was still waiting for the numbers for the final day of the campaign on Thursday.

He said WHO was "satisfied" with the campaign, hailing the large numbers of parents who had turned out to ensure their children were protected against polio.

Poliovirus, most often spread through sewage and contaminated water, is highly infectious. It can cause deformities and paralysis, and is potentially fatal. It mainly affects children under the age of five.

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

The WHO has stressed that it is vital to reach at least 90 percent coverage to avoid the spread of the disease both within Gaza's borders and beyond.

Peeperkorn said the WHO was "very much grateful that the area-specific humanitarian pauses have been respected" during the first phase of the campaign, which he said had been permitted to take place in "a polio bubble".

He called for the respite afforded to be extended to "a much broader area", allowing for the establishment of proper humanitarian corridors to ensure aid can reach those in need.

Israel's assault on Gaza has killed at least 41,118 Palestinians and wounded more than 95,125 others, according to the territory's health ministry. The UN rights office says most of the dead are women and children.

*This story was edited by Ahram Online.