Nigeria has announced an emergency mass polio vaccination campaign in the north-east after two new cases emerged.
They were the first incidences of the highly infectious disease in Africa for more than two years.
The government said polio paralysed two children in Borno state, a part of Nigeria where Boko Haram militants have hindered health campaigns.
The development is seen as a major setback for Nigeria, which was on track to be declared polio free in 2017.
The cases were confirmed exactly two years after Africa’s last previous case – in the Puntland region of Somalia, on 11 August 2014.
Nigeria’s government said that one million children would be immunized in the affected areas in Borno and a further four million will also be targeted in neighbouring states.
Health Minister Isaac Adewole said the priority was to “boost immunity” and “ensure that no more children are affected by this terrible disease”.
The Nigerian authorities, along with experts from the World Health Organization (WHO), are currently investigating the situation to find out where the virus has spread.
The mass vaccination could start “as soon as next week”, WHO Director for Polio Eradication Michel Zaffran told the BBC Newsday programme.
The militant Islamist Boko Haram insurgency in north-east Nigeria has made some areas of Borno hard to access in recent years.
The fact that these two cases have been discovered was a result of increased health surveillance in the north-east, which was made possible by military success against Boko Haram, the government said.
Source: World Health Organization
Nigeria has made a lot of progress in reversing the spread of polio in the last five years.
In 2012, the country had more than half of all polio cases worldwide, the WHO says.
The WHO puts the reduction down to a “concerted effort by all levels of government, civil society, religious leaders and tens of thousands of dedicated health workers”.
In order for the WHO to declare a country free of polio it has to go three years without a new case.
Polio is a viral disease that usually affects children and can only be prevented through immunisation.
It is spread by poor sanitation and contaminated water.