The World Health Organization said on Wednesday that a mass campaign to vaccinate children against polio in Gaza would resume on Saturday, with over half a million children targeted.

"The current environment in Gaza, including overcrowding in shelters and severely damaged water, sanitation and hygiene infrastructure, which facilitates fecal-oral transmission, create ideal conditions for further spread of poliovirus," the WHO statement said.

"Extensive population movement consequent to the current ceasefire is likely to exacerbate the spread of poliovirus infection," it added.

In September and October last year WHO conducted two rounds of vaccinations across the enclave, reaching over 95% of the target.

The virus is still circulating in the enclave, meaning it could still reach a child who is under or un-immunized. Six samples tested positive in December and January, WHO added.

Though access for health workers has improved since the ceasefire came into place on January 19, some areas could not be accessed due to the fighting at the time. 7,000 children missed vaccination in Jabalia, Beit Lahiya, and Beit Hanoun, according to the organisation.