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 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.
Why did polio return? By Anne Soy, BBC Africa health correspondent
Just when Africa thought it had gone two years without a case of wild polio virus, the debilitating disease reared its head again.
The disappointment was immediately clear.
Nigerian Health Minister Isaac Adewole described the two new cases as a setback.
WHO's Regional Director for Africa Matshidiso Moeti said she was deeply saddened.
But how could the virus have "disappeared" for two years before re-emerging?
WHO said there was possible low-level transmission during that period.
About one in 200 cases of infection results in paralysis.
As long as there are unvaccinated children, there will be a good chance of it surviving and spreading.
The answer for now is to vaccinate all children under five.
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.
What is polio?
- Polio, or poliomyelitis, mainly affects children aged under five
- It is a highly infectious disease caused by a virus. It invades the nervous system and can cause total paralysis in a matter of hours
- Initial symptoms include fever, fatigue, headache, vomiting, stiffness of the neck and pains in the limbs
- One in 200 infections leads to irreversible paralysis. Among those paralysed, 5% to 10% die when their breathing muscles become immobilised
- Today, only two countries - Afghanistan and Pakistan - remain polio-endemic, down from more than 125 in 1988
Source: World Health Organization