YOBE, Nigeria -- Nigerian troops operating in northeast Yobe State on Saturday said 21 persons were killed by gunmen suspected to be Boko Haram members at Government Secondary School Mamudo in Potiskum Local Government Area of the state.

Spokesperson for the Joint Task Force Lt. Lazarus Eli, who confirmed the attack, told Xinhua in Damaturu, the state capital that 20 students and a teacher were on Saturday killed by the gunmen, noting that four other students were now lying critically ill in a hospital.

He said the attack took place at around 5.30 a.m, local time.

Local police sources and residents however told Xinhua that about 28 college students and a teacher were killed in an attack on a public school.

The police source said the Boko Haram invaded the Government Secondary at Mamudo, a small community along Potiskum-Damaturu road in Yobe late Friday night.

Residents also said the attackers opened fire on the students in their hostels, killing at least 28 of them while a teacher was also victim.

"They subsequently set the hostel on fire," Illiyasu Ado, a resident and commercial driver said. He said some students  sustained injuries during the attack.

Residents said they heard heavy gun shots in the midnight and never thought their children in the only secondary school are the targets of the attack.

They wondered how the attackers could storm the school despite the 6 p.m, local time to 6 a.m, local time curfew in the area.

Sources said military troops have already condoned off the town. Residents feared attacks on schools, students and teachers could go on for long except the military put on adequate measures to curtail the security situation.

The two northeast states of Yobe and Borno, worse hit by the insurgency and the neighboring Adamawa State, also in the northeast are currently under state of emergency imposed by Nigeria President Goodluck Joanthan.

Additional troops and hard wares were also deployed to the states to complement existing Joint Task Force (JTF ).