POTISKUM Nigeria (Reuters) - A suicide bomber dressed as a student killed at least 48 people, most of them students, and injured 79 others at a school assembly in the northeastern Nigerian town of Potiskum on Monday, a hospital official said.

No one immediately claimed responsibility for the attack in Yobe State, a territory close to the stronghold of Sunni Muslim Boko Haram militants, who have staged a five-year insurgency.

A Reuters reporter in Potiskum said angry locals had blocked access to the school and an adjoining hospital, preventing security forces from getting close to the site of the explosion.

People in the crowd did not want a repeat of what happened last week when members of the security forces fired at residents after a bomb killed nearly 30 people, a police source said. [ID:nL6N0ST33A]

Boko Haram has intensified its attacks since the Nigerian government announced a ceasefire agreement last month as well as the imminent release of more than 200 school girls kidnapped by the group. Boko Haram's leader denied a ceasefire deal had been reached and the school girls have not been set free.

The group whose name means "Western education is sinful" in Hausa, has attacked schools, abducted hundreds of students and killed thousands in its fight for an Islamist state, and is seen as the main security threat to Africa's leading oil producer.

"So far, the number of the dead is 48, while 79 are injured. I counted the bodies, mostly students and a few teachers," a nurse at Potiskum General Hospital told Reuters.

"A teacher who survived the blast with minor injury said the bomber dressed like a student and was also on the assembly ground with the students," she said, asking to remain anonymous.

The Nigeria Police Force confirmed that the bomber was disguised as a student and said 47 people had been killed.

Mariam Ibrahim, a teacher at the Government Science Secondary School (GSS) in Potiskum told Reuters the bomb went off as she was arriving and students were at morning assembly.

Potiskum resident Aliyu Abubakar said he heard the explosion when he was dropping off his two sons at a nearby Islamic college. "One of my sons fell down, I came out dragged him in and we drove off back home," he said.

A second teacher, asking to remain anonymous, said, "There are some (others) that are critically injured and I am sure the death toll will rise."