The UN on Friday said it fears a surge in polio cases among children who have escaped from the jihadist bastion of Fallujah, and has launched a "massive" vaccination campaign.

Residents of Fallujah, which Iraqi forces backed by US-led air strikes are pushing to recapture from the Islamic State group's control, are suffering from extremely high rates of skin disease, hyper tension and diarrhoea, said Ala Alwan of the World Health Organization.

Speaking to journalists by phone after touring camps for displaced people around Fallujah, Alwan said mothers were nervous because their children had not been vaccinated since the IS takeover in 2014.

"A specific concern for us is polio," said Alwan, the WHO's chief for the eastern Mediterranean region.

"We have started a massive vaccination programme," he said, urging donor nations to boost their support for Iraqi civilians fleeing the fighting.

He said it was too early to estimate the number of children to be targeted in the vaccination drive.

Fallujah, which lies just 50 kilometres (30 miles) west of Baghdad, is one of IS' most emblematic strongholds.

Iraqi forces said they retook the main government compound in the city earlier on Friday.

Alwan estimated that 40,000 people had fled the city during the offensive and that another 30,000 to 40,000 "are still inside."

Camps for the displaced are filling up and more capacity is urgently needed, he told reporters.

"We have a huge demand," Alwan said. "It's very, very sad situation."