There are now six confirmed cases of polio in Papua New Guinea, with two more identified in the past week, one in Madang and the other in Eastern Highlands.
PNG, which had been declared free of polio for 18 years, found a child in Morobe with the disease in June.
Systematic vaccination programmes have since revealed more victims and dozens of others with a suspected polio infection.
The government said the risk of further spread within the country is high.
Working with the WHO, UNICEF and the Global Polio Eradication Initiative, PNG health officials are vaccinating hundreds of thousands of children throughout the country over the coming weeks, starting in six provinces today.
They are Enga, Western Highlands, Southern Highlands, Chimbu, Jiwaka and Hela.