The World Health Organization on Friday declared Pakistan's troubled northwestern city of Peshawar as the world's "largest reservoir" of endemic polio and called for urgent action to boost vaccination.
Almost every polio case in 2013 in Pakistan, one of only three countries where the crippling disease remains endemic, could be linked genetically to strains of the virus circulating in Peshawar, said the WHO.
Khyber Pakhtunkhwa province, of which Peshawar is the capital, and adjoining tribal districts where Taliban and Al-Qaeda linked militants have hideouts are particular polio hotspots.
Efforts to stamp out the disease have been hampered by opposition from militant groups, who see vaccination campaigns as a cover for espionage, as well as long-running rumours about the drops causing infertility.
"With more than 90 percent of the current polio cases in the country genetically linked to Peshawar, the (city) is now the largest reservoir of endemic poliovirus in the world," the WHO said in a statement.
Polio is also endemic in Afghanistan and Nigeria, but of the three countries only Pakistan saw a rise in cases from 2012 to 2013, said the global health body.
Tests have found that 83 out of the 91 polio cases in Pakistan last year were genetically linked to strains in Peshawar, while 12 out of 13 cases reported in Afghanistan were also linked.
In November the WHO linked an outbreak of the disease that paralysed 13 children in war-torn Syria to Pakistan.
Friday's announcement comes just days after Pakistan's neighbour and great rival India celebrated three years since its last polio case.
India's polio programme is one of its biggest public health success stories—a remarkable achievement for a country that only five years ago accounted for half of the world's polio cases—and it stands in stark contrast to Pakistan's faltering efforts to combat the virus.
During the last six months, every single sample taken from Peshawar's sewage system tested positive for a highly contagious wild polio virus strain, the WHO said.
A major outbreak of polio in the Federally Administered Tribal Areas (FATA) along the Afghan border, which has left 65 children paralysed, is being "sustained" by Peshawar, it added.
The city of four million is the main urban centre of northwest Pakistan and acts as a major transit hub.
"As much of the population of the area moves through Peshawar, the city acts as an amplifier of the poliovirus," said the WHO.
Militants in Pakistan have routinely attacked health workers giving out polio drops since the Taliban imposed a ban on vaccinations in 2012.
In the most recent incident gunmen shot dead a health official supervising an anti-polio drive after storming a hospital on the edge of Peshawar on December 28.
"The prevailing security situation in Peshawar has seriously affected the quality of polio campaigns in the city and is resulting in inadequate coverage of children against the virus," said the health organisation.
Elias Durry, the WHO's emergency coordinator for polio eradication in Pakistan, told AFP the virus was concentrated in a few areas that were too dangerous for health workers to visit.
For eradication to succeed, he said, security must be improved in these areas.
Last month Pakistani cricketer-turned-politician Imran Khan, whose Pakistan Tehreek-e-Insaaf party leads the Khyber Pakhtunkhwa provincial government, pledged to personally spearhead anti-polio efforts.
He kick-started the campaign by administering polio drops to the grandson of radical cleric Maulana Sami-ul-Haq, known as the "Father of the Taliban".
The cleric has issued a fatwa urging parents to immunise their children against polio, saying vaccinations complied with Islamic law.
Provincial health minister Shaukat Ali Yousafzai told AFP: "We are committed to eliminating the disease and doing our best despite continuous attacks on polio workers."