Gunmen believed to be Boko Haram Islamists attacked a secondary school in Nigeria's restive northeast yesterday, killing 42 people, many of whom were students, a medical worker and residents said.

But a military spokesman said 20 students and one teacher were killed in the dawn attack at Yobe state.

The attackers rounded up students and staff of the school and placed them in a dormitory before throwing explosives inside and opening fire, said Haliru Aliyu of Potiskum General Hospital, quoting witnesses who escaped.

"We received 42 dead bodies of students and other staff of Government Secondary School (in) Mamudo last night. Some of them had gunshot wounds," Aliyu told AFP, adding that many also had burns.

Aliyu said security personnel were combing the area around the school in search of wounded students who have fled the attack.

"So far six students have been found and are now in the hospital being treated for gunshot wounds," he said.

Mamudo is some five kilometres (three miles) from Potiskum, the commercial hub of the state of Yobe, which has been a flashpoint in the Boko Haram insurgency in recent months.