The World Health Organization added that the first round of the vaccination drive would take another 10 days.

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 -- disease has spread.

After the first confirmed polio case in 25 years, a massive vaccination effort began on Sunday, with localised "humanitarian pauses" in fighting.

The campaign aims to fully vaccinate more than 640,000 children in the besieged territory, devastated by almost 11 months of war.

Mainly affecting children under five, polio can cause deformities, paralysis and in some cases death.

Rik Peeperkorn, the WHO's representative for the Palestinian territories, said it was vital to reach at least 90-percent coverage to avoid the spread of the disease both within Gaza's borders and beyond.

The campaign began in the central part of the densely populated Gaza Strip, where the WHO initially expected to vaccinate 156,500 children under the age of 10.

"Our target for the central zone was an underestimation," Peeperkorn said, adding this was probably due to more people being crowded into the area than anticipated.

He said the vaccination drive was expected to shift to southern Gaza on Thursday, with the aim of immunising some 340,000 children there.

It would then move to the north of the strip, where around 150,000 will be vaccinated.

"We still have 10 days to go at least" for the whole first portion of the campaign, Peeperkorn said, and the rollout of the necessary second dose would begin in four weeks' time.

- 'Extremely concerned' -
As he related his visit to a health centre handing out the vaccine, Peeperkorn said he was "not even so surprised" the campaign had gotten off to a good start.

"There were so many -- the fathers, mothers -- bringing their children in, and children really proud and happy that they got vaccinated."

He pointed to Gaza's "high vaccination acceptance" with pre-war routine vaccine coverage of between 90 and 95 percent, "which is actually much better than a lot of high income countries."

But the WHO representative warned the agency was "extremely concerned" by Gaza's wider health situation.

With only 16 of 36 hospitals operational, the strip has seen a "huge increase in infectious diseases".

"We've seen more than a million, mainly children, diagnosed with acute respiratory infection," Peeperkorn said, adding that more than 600,000 children had suffered from diarrhoea.

While polio vaccinations are best carried out in house-to-house campaigns, Peeperkorn said that those are impossible in Gaza as "there's very few houses left and people are everywhere".

Since Oct.7, Israel launched a brutal military assault on the Gaza Strip which has so far killed at least 40,819 people, the majority of them women and children, with over 94,291 others wounded, according to the Palestinian health ministry.

* This story was edited by Ahram Online.