A suspected Boko Haram suicide bomber disguised in school uniform killed 47 students in northeast Nigeria Monday, prompting US and UN condemnation of one of the worst attacks against schools with a so-called Western curriculum.

The explosion ripped through an all-boys school in Potiskum just as students gathered for morning assembly before classes began, causing panic and chaos.

The massacre, described by President Goodluck Jonathan as a "dastardly attack", came just a day after the release of a new Boko Haram video in which the Islamist group's leader, Abubakar Shekau, again rejected Nigerian government claims of a ceasefire and peace talks.

UN Secretary-General Ban Ki-moon called on Nigerian authorities to bring the perpetrators of the school attack to justice.

The UN chief "is outraged by the frequency and brutality of attacks against educational institutions in the north of the country", his spokesman said.

US State Department spokeswoman Jen Psaki echoed the call for the perpetrators to be brought to justice and condemned "the horrific attack" by "those bent on fomenting violence, extremism and insecurity".

Students at the Government Comprehensive Senior Science Secondary School were waiting to hear the principal's daily address when the explosion happened at 7:50 am (0650 GMT).

Several witnesses said the blast was "thunderous" and described a bloody aftermath of abandoned footwear, charred school books, bags and body parts.

"There was an explosion detonated by a suicide bomber. We have 47 dead and 79 injured," national police spokesman Emmanuel Ojukwu said, adding that Boko Haram was believed to be responsible.

Ojukwu also confirmed local media reports that the bomber was disguised as a student at the school, which has more than 1,000 pupils aged 15 to 20.

Yobe state governor Ibrahim Gaidam ordered the immediate closure of all public schools in the Potiskum area and slammed the government in Abuja for failing to tackle the insurgency.

"Urgent action must be taken right now to restore a fast-waning public confidence by doing whatever it takes to stop the escalating violence," he said in a statement.

Jonathan, who is expected on Tuesday to formally declare his bid for re-election, promised those affected that his government would find the culprits, "no matter how long it takes".

Blood-stained uniforms

Student Adamu Abdullahi said pupils at the centre of the blast near the principal's office were flung in all directions and others were knocked off their feet.

"I found myself under the weight of another student, who fell over me. I'm certain he was dead. I was dazed and disorientated for a moment," he told AFP.

"When I realised what had happened, I managed to push the body on top of me and started running like everyone else. It was confusion all over. Everybody was hysterical.

"I saw many people on the ground. Human flesh and blood were splattered all over the place... I ran out of the school and went home.

"When my father saw me he was terrified. I didn't realise my white school uniform was stained with human blood and bits of flesh."

The dead and injured were taken to the Potiskum General Hospital just 100 metres (yards) away.

Boko Haram, which wants to create a hardline Islamic state in northern Nigeria, has previously carried out deadly attacks on schools teaching a curriculum it deems to be Western.

In February, gunmen killed at least 40 students after throwing explosives into the dormitory of a government boarding school in Buni Yadi, also in Yobe state.

In July last year, 42 students were killed when Boko Haram stormed dormitories in a gun and bomb attack on a government boarding school in the village of Mamudo, near Potiskum.

Boko Haram's most high-profile attack on a school came in April, when fighters kidnapped 276 girls from the town of Chibok in Borno state, also in northeast Nigeria.

More than six months later, 219 of the girls are still being held.

Relentless violence

Potiskum has been repeatedly targeted in deadly attacks blamed on Boko Haram, including last Monday, when a suicide bombing killed at least 15 at a Shiite religious ceremony.

Yobe is one of three northeastern states that has been under a state of emergency since May last year.

But violence has continued unabated and Boko Haram has seized at least two dozen towns and villages in recent months, raising doubts about the government's ability to control the region.