GENEVA: About 50,000 Pakistanis have crossed into eastern Afghanistan to escape air strikes over the past 10 days and 435,000 have fled within their homeland, which could fuel the spread of polio as many are not vaccinated, UN agencies said on Tuesday.

Pakistan's military gave residents of North Waziristan until the end of Monday to leave the remote mountainous region ahead of a widely anticipated major ground offensive by the army against militants.

The local governing shura, or religious leadership in North Waziristan, has banned polio vaccination for the past two years, demanding a halt to US drone strikes, according to the World Health Organization (WHO).

The number of Pakistanis who have fled to Afghanistan since military operations began on June 15 jumped from 7,000 last Friday to an estimated 50,000, Dan McNorton of the UN refugee agency told a briefing. Most are in Khost and Paktika provinces.

“There was a temporary halt to military operations, to air strikes on Sunday ... So that why we have seen the figures rise particularly quickly in the last couple of days,” McNorton said.

Most of the 435,000 displaced within Pakistan are in Bannu, Lakki Marsat, Dera Ismail Khan and Tank in Kyber Pakhtunkhwa province, but some arrived in Punjab and Balochistan, he said.

The ban on vaccinations in North Waziristan has led to a “huge outbreak of polio”, accounting for 53 of the country's 82 cases recorded so far this year, WHO spokeswoman Sona Bari said.

Last year Pakistan had 93 cases of polio, a crippling disease that invades the nervous system and can cause irreversible paralysis within hours. There is no cure but it can be prevented by immunisation. Children are the most vulnerable.

“So there is a risk that now with the population moving out (of North Waziristan) that the virus will move out,” Bari said.

But the exodus from North Waziristan also represents the first opportunity to vaccinate these people in two years, she said, noting that Afghan authorities and Pakistan authorities in bordering districts were trying to vaccinate those fleeing.

Afghanistan has recorded six cases of polio this year among a total of 103 polio cases in nine countries, according to the WHO which spearheads a global campaign to wipe out the disease.