Nigeria's President Muhammadu Buhari has said he is surprised by the rising threat of the Islamist militant group Boko Haram in the northern regions of the country.

“I was taken aback by what is happening in the north-west and other parts of the country. During our campaigns, we knew about the Boko Haram. What is coming now is surprising. It is not ethnicity or religion, rather it is one evil plan against the country," Mr Buhari is quoted as saying by the Vanguard site.

He told a visiting delegation from Niger state that the government would enhance its crackdown on the group.

“We have to be harder on them. One of the responsibilities of government is to provide security. If we don’t secure the country, we will not be able to manage the economy properly,” the Punch newspaper quoted Mr Buhari as saying.

On Monday he ordered air strikes against bandits, kidnappers and cattle rustlers in forested areas bordering Niger, Kaduna and Zamfara states.

Over the weekend, gunmen killed 11 people and kidnapped four women during door-to-door attacks in Niger state, reported Sahara Reporters and The Daily Sun.