A bombing at a football pitch in Nigeria's restive northeast killed at least 40 people on Sunday in an area previously attacked by Boko Haram Islamists, a police officer and a nurse said.

The blast hit the town of Mubi in Adamawa state, one of three in the northeast which has been under a state of emergency for more than a year as Nigeria's military has tried to crush Boko Haram's five-year extremist uprising.

"There has been a bomb explosion at a football field this evening and so far more than 40 people have been killed," said the officer in Mubi who requested anonymity.

The policeman's account was confirmed by a nurse at the Mubi General Hospital, who also requested anonymity, as the health worker was not authorised to discuss the attack with the media.

Adamawa has been hit by far fewer Boko Haram attacks than other parts of the northeast, but the town was the site of a gruesome October 2012 massacre at a post-secondary technical college.

Scores of students were killed in their dorms, including many whose throats were slit.

Mubi is just a few kilometres from Nigeria's border with Cameroon and near the area where two Italian priests and a Canadian nun were seized by suspected Boko Haram gunmen in April.

The three were released earlier on Sunday and flown out on board a military aircraft from the town of Maroua, heading for the Cameroon's capital.

- 'Women and children caught in the blast' -

The policeman in Mubi said the bomb exploded at roughly 6:30 pm (1730 GMT) at the pitch in Mubi's Kabang area and targeted fans who were trying to leave the field after watching a local club match.

It was not immediately clear if players were among the casualties, but the officer and the nurse said it appeared most of the victims were fans.

Muhammad Hassan, a Mubi resident who was at the match, said the blast appeared to come from within the crowd of people who were walking across the pitch and heading home after the final whistle.

"Women and children were caught up in the blast," he said.

Boko Haram has carried out scores of attacks on targets it says are a product of Western influence, including sports venues and schools teaching a secular curriculum.

The group has killed thousands during its battle against the government since 2009, but the conflict has received unprecedented global attention over the last six weeks following the Islamists' mass kidnapping of more than 200 schoolgirls.

The girls were seized on April 14 from Chibok in Borno state, which shares a border with Adamawa.

Nigeria's response to the kidnapping has been fiercely criticised as inept and the crisis has piled intense pressure on President Goodluck Jonathan's government to do more to end the uprising.

The United States, Britain, France and Israel are providing varying levels of support to Nigeria's rescue mission for the 219 girls being held by the extremists.

While early suspicion for the latest attack fell on the Islamist insurgents, the eastern part of Nigeria has also seen sectarian violence at football pitches not thought to be linked to Boko Haram.

In Taraba state, just south of Adamawa, scores of people were killed last year in fighting between supporters of a nominally Muslim club and mostly Christian fans of a rival club. (AFP)