Pakistan, one of only four countries where polio is still endemic, will launch a nationwide vaccination drive Tuesday aiming to inoculate 32 million children against the disease.
Efforts will be concentrated on the border regions near Afghanistan, where ease of movement between the two countries is seen as especially problematic in curtailing the spread of the disease, health officials said.
Around 75,000 vaccination teams, with 150,000 workers, will fan out across the country for three days to administer polio drops to children aged under five, they said.
Vaccination efforts had been hampered in the past, he told AFP, because community leaders in the tribal regions rarely cooperated with people they regard as "outsiders."
And while some tribal elders had given assurances that vaccination teams would be welcomed, WHO still faced difficulties gaining access to what he called "high-risk" tribal areas, including North Waziristan, Bajaur and Khyber.
The three regions are virtually outside federal governance and controlled by an age-old system of tribal leadership that is deeply conservative and religious, suspicious of outsiders, and rarely cooperative with central government initiatives.
Nevertheless, the health authorities have been working hard to win the cooperation of tribal elders, with education drives aiming to convince them of the merit of vaccination against such debilitating diseases as polio.
As a result, the incidence of polio in Pakistan has been declining in recent years, with 11 new cases detected this year, compared to 40 in all of 2006.
WHO officials said most of the cases had been detected in areas considered uncooperative -- two in southwestern Baluchistan province, one in the Khyber tribal district, and four each in North West Frontier Province and southern Sindh province.
Despite the fall in detected cases, alarm bells were sounded last month when a 22-year-old Pakistani man studying in the southern Australian city of Melbourne was found to be infected with polio.
"This boy had contracted the virus during a visit to North West Frontier Province early this year," a senior health ministry official said.
Pakistan stands with neighbouring Afghanistan, India and Nigeria as one of four countries where polio is endemic.
The number of annual infections in Pakistan was 20,000 to 30,000 in the early 1990s before the inoculation drive, known as the Expanded Programme on Immunization, was launched in 1994.
WHO's Islam said a total of 337 cases had been detected in 10 countries around the world this year, including 11 in Pakistan, 124 in India, 138 in Nigeria and five in Afghanistan.
The remaining 59 cases were detected in Somalia (8), Myanmar (11), Democratic Republic of Congo (27), Niger (5), Angola (6) and Chad (2), all non-endemic countries.
Worldwide cases in 2006 totaled 1,998, according to WHO figures.
Islamabad aims to eradicate the disease by 2010 but officials said the fluidity of movement across the border with Afghanistan made it particularly difficult to check the spread of the disease in the rugged border regions.
Health Ministry official Mazhar Nisar Sheikh said border posts had been set up, with help from the WHO, in an effort to reach children who live on the border and have regular contact with people from the Afghan side.
It was "an uphill task" to check polio transmission among these children, he said, adding that 10 checkpoints had been set up along the western border.
This will be the second polio vaccination campaign in the last seven months. The government plans to launch two more nationwide drives in October and December.