At least 31 people have been killed in the Nigerian city of Yola, capital of Adamawa state, during an attack carried out by Boko Haram. Officials told AP that 38 people have been injured in the bomb blast.

The latest attack occurred shortly after another bomb blast at a market in Maiduguri, the capital of the Nigerian restive state of Borno, killed 50 people.

Who are Nigeria's Boko Haram terrorists?

Boko Haram fights against Western influence in Nigeria and aims to impose its version of Sharia law on the country. The group declared an Islamic caliphate in Gwoza, along the Cameroon border, in August 2014.

Boko Haram has raided several cities in the north of the country in a bid to take control of more land.

Three states − Adamawa, Borno and Yobe − have been under a state of emergency since May 2013, due to Boko Haram's attacks.

The bombing follows a video released by the terror group in which an unidentified masked man dismissed claims by the Nigerian government that it had recaptured most of the areas invaded by Boko Haram.

"The armies claim through the media that they captured our towns and that they assaulted Sambisa and defeated us," he said.

The man was referring to the government's announcement that the army had entered what has been deemed "the terrorists' last-known stronghold" in the Sambisa forest, on the border with Cameroon.

The Nigerian government is being aided by mercenaries and troops from neighbouring countries Chad, Benin, Niger and Cameroon in its offensive and has scored some successes since the military cooperation started in February.

The US announced it would send military trainers to help the Nigerian army improve its intelligence gathering and logistics.