KANO, Nigeria (AFP) - The death toll in a suspected Boko Haram suicide bombing of a secondary school in north-east Nigeria has risen to 58, a school official told AFP on Wednesday.

"From records at our disposal, the death toll now stands at 58 while the number of those injured has soared to 117," said the official at the school in Potiskum, Yobe state.

A bomber disguised in school uniform blew himself up as students gathered for morning assembly at the Government Comprehensive Senior Science Secondary School before classes began on Monday.

Federal police previously gave the death toll as 47 with 79 injured and said that Boko Haram, which is opposed to so-called Western or secular education, was believed to be responsible.

The attack was one of the worst against a state-run school in Boko Haram's bloody five-year insurgency, which has seen boarding school students killed in their beds.

The group was also responsible for the mass kidnapping of 276 schoolgirls from the northeastern town of Chibok in Borno state in April. More than six months on, 219 are still being held.

The school official asked to remain anonymous because he was not authorised to talk to the media.

He said the higher toll was because many of the dead and injured were taken straight home rather than to hospital until an appeal was made by the state government.

President Goodluck Jonathan, announcing his bid for re-election on Tuesday, described the Potiskum attack as "a dark cloud on our nation" but vowed to defeat the insurgents.