The United Nations says it is gravely concerned about a spate of attacks by the jihadist group Boko Haram in north-east Nigeria that are reported to have killed close to 40 people in recent days.

The UN's humanitarian coordinator in Nigeria, Edward Kallon, called on the government and the security forces to do more to protect communities.

Boko Haram was founded in 2002 and launched military operations in 2009.

Its insurgency has killed thousands and displaced millions mostly in the north-east of the country.

The militant group has also extended its activities into neighbouring countries.

Mr Kallon also urged the Nigerian authorities in Kaduna state to investigate last week's clashes in the Kasuwan Magani area which left more than 50 people dead.

The governor of Kaduna state, Nasir Ahmad El Rufai, says the prosecution of 25 people arrested over the violence will be fast-tracked.