KANO, Nigeria (AFP) -- Boko Haram gunmen on Sunday opened fire in a college dormitory in northeast Nigeria as the students slept, killing at least 28 people, in the latest violence blamed on the Islamist insurgents.

The early morning assault targeted the College of Agriculture in the town of Gujba, some 30 kilometres (18 miles) from Damaturu's capital of Yobe, said area military spokesman Lazarus Eli.

"Boko Haram terrorists...went into the school and opened fire on students" while they were sleeping, he told AFP.

An official at the Damaturu Specialist Hospital, who requested anonymity, said "28 bodies" had been received so far from the Gujba massacre.

"We have been asked to expect more," he told AFP.

Yobe state, where the college is located, has seen a spate of attacks on schools and universities, all blamed on Boko Haram, an extremist group that has killed hundreds in its four-year insurgency.

The name Boko Haram means 'Western education is forbidden' and the group has become notorious for slaughtering students of various ages in recent years.

In July in the town of Mamudo in Yobe, Islamists threw explosives and sprayed gunfire into dormitories in the middle of the night, killing 41 students.

Presumed Boko Haram gunmen shot dead seven secondary school students and two teachers in Damaturu in June.

The military has described the recent school attacks as a sign of desperation by the Islamists, claiming they only have the capacity to hit soft targets.

The defence ministry has said that an offensive launched against Boko Haram in mid-May has decimated the group and scattered their fighters across remote parts of the northeast, the insurgents' traditional stronghold.

While many of the recent attacks have occurred in more remote areas, often targeting defenceless civilians, the unchecked killing has cast doubt on the success of the military's campaign.

Scores have been killed this month, including in the northeastern town of Benisheik in Borno state, where at least 142 people were slaughtered by presumed Boko Haram fighters who came disguised as soldiers, set up checkpoints and fired on motorists and bystanders.

Some of the recent violence has targeted vigilante groups which have formed to help the military.

The northeast remains under a state of emergency imposed on May 14.

The phone network has been switched off in much of the northeast since the emergency measures were declared, a move the military said would help prevent the Islamists from coordinating attacks.

Some have suggested that the lack of phone service has prevented civilians from sounding the alarm during attacks.

While the phones remained down in Gujba, calls to Damaturu were going through on Sunday.

Boko Haram has said it is fighting to create an Islamic state in Nigeria's mainly Muslim north, but the group is believed to made up of different factions with varying aims.

The group has attacked churches, mosques, newspaper offices, the security forces, politicians and a United Nations building, among other targets.

A toll earlier this year said the conflict is estimated to have cost more than 3,600 lives, including killings by the security forces. The current figure is likely much higher.

Nigeria is Africa's most populous country and top oil producer, roughly divided between a mainly Muslim north and predominately Christian south.