The Nigerian government has reported two cases of polio - the country's first in two years.

It said two children had been paralysed by the disease in the north-eastern Borno state, a part of Nigeria where Boko Haram militants are active.

The development is seen as a major setback for Nigeria, which was on track to be declared polio free in 2017.

Polio is an infectious viral disease that usually affects children. It can only be prevented through immunisation.

The disease is spread by poor sanitation and contaminated water.

A UN immunisation programme in Nigeria has had to contend with rumours that the vaccine was a plot to sterilise Muslims.

Polio vaccination centres in the country have also been attacked by Boko Haram Islamist militants.