A bomb has exploded outside a stadium in Nigeria's Gombe city, minutes after President Goodluck Jonathan had left a campaign rally there.

At least one bystander was killed and seven others wounded when the car bomb exploded, Reuters news agency reports.

Mr Jonathan is standing for re-election in presidential elections due to be held on 14 February.

Militant Islamist group Boko Haram has stepped up attacks ahead of the polls.

Mohammed Bolari, who was at the rally, said the explosion occurred some three minutes after Mr Jonathan's departure, AFP news agency reports.

"The president had just passed the parking lot and we were trailing behind his convoy when the explosion happened," he is quoted as saying.

Mr Jonathan addressed a rally in the north-eastern city a day after it was hit by two blasts that killed at least five people, AFP reports.