The World Health Organization (WHO) announced on Friday that Pakistan's city of Peshawar is the world's "largest reservoir" of polio virus and called for an immediate action to boost vaccination efforts in the region.

"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.

Pakistan is one of the three remaining countries in the world where polio remains endemic and the only one that experienced a rise in polio cases. From 58 in 2012, polio cases in the country rose to 91 last year. Pakistan is also the only country that spreads the virus across borders. The WHO reported that almost all of the polio cases in Pakistan can be genetically linked to Peshawar and 12 of 13 cases reported in Afghanistan last year can also be traced back to the city.

Peshawar, the capital city of Khyber Pakhtunkhwa province, serves as a major transit hub for thousands of people a day, increasing risks of transmission. Pakistanis seeking refuge from tribal wars also go to the city, bringing with them the infectious virus when they leave. "As much of the population of the area moves through Peshawar, the city acts as an amplifier of the poliovirus," WHO said.

Vaccination is apparently the most feasible solution to end the disease as aggressive vaccination campaigns have eradicated polio in all but three countries in the world: Nigeria, Afghanistan and Pakistan but vaccination efforts have proved to be difficult. Militant groups prevent vaccination teams from accessing some areas believing that vaccination campaigns are cover for espionage. Some religious leaders also spread false claims that the vaccine is meant to sterilize Muslim children.

"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," the WHO said.

For some, the effort to save people from the disease can be life threatening as well. Health workers giving out polio vaccines are routinely attacked with one health official supervising an anti-polio drive gunned down. Elias Durry, the WHO's emergency coordinator for polio eradication in Pakistan, said that law and order must be improved in dangerous areas to ensure success of eradicating polio. ''If you do not take care of this reservoir it will keep threatening other parts of the country as well. Then the final eradication would be much more difficult,'' Durry told The Associated Press.