More than 250,000 health workers have fanned out across Pakistan to vaccinate millions of children against polio in a week-long drive, in the country's push against the crippling disease.

The drive would target 38.6 million children under the age of five, said Safdar Rana, who leads Pakistan's fight against polio.

The children of Afghan refugees who frequently travel between the two countries would also be administered the vaccine, Rana added.

Pakistan and Afghanistan are among only a few countries in the world where polio still exists as an epidemic.

The United Nations-funded vaccination campaign has, however, helped Pakistan to control the spread as the number of new affected children came down to only four so far this year compared to 306 in 2014.

"Pakistan is closest ever to its goal (of polio eradication)," Rana said.

The disease that cripples kids for life spread in the country's mountainous north-west near the Afghan border, which remained under the control of militants linked with al-Qaeda for several years.

The militants have killed hundreds of health workers and police guarding them because they thought the polio vaccine is intended to make Muslim children sterile.

A series of military offensives was launched against Islamist militants to make sure workers could reach children in previously inaccessible areas.