Nigeria's military was given early warnings of a Boko Haram attack on a boarding school in Chibok from where more than 200 schoolgirls were abducted, according to sources independently verified by Al Jazeera and Amnesty International.

Two politicians from Nigeria's northeast Borno state told Al Jazeera's Yvonne Ndege that security forces had been given at least two hours' warning of an attack on the town, but failed to act.

This is a shocking revelation. Makmid Kamara, Nigeria researcher, Amnesty International

Makmid Kamara, a Nigeria researcher for Amnesty International, said the group had been told the military had at least four hours' advance warning.

The schoolgirls remain missing after being abducted on April 14, more than three weeks ago. The Nigerian government's slow response to the abduction has led to protests around the country.

"We received information and we spoke to a senior Nigerian military officer, who spoke to us on condition of anonymity, that they had received intelligence reports, even before local authorities and politicians got the information, that gunmen were on their way to the Chibok town," Kamara said.

Kamara told Al Jazeera that senior officials in Maiduguri and Dambua towns were alerted around 7pm on April 14, and that information was given to the senior military officers based in Dambua and Maiduguri.

"Later on, at 10pm on the same night of the 14th of April, local authorities, who Amnesty had spoken to, informed us, that they informed the local military command in Chibok town about the planned attack," Kamara said.

"When I spoke to one of the senior military officials, they informed me that they [had] informed their superiors, and requested for reinforcement. But the reinforcement did not come."

17 soldiers against Boko Haram

Kamara said despite the warning to assemble reinforcements, only 17 soldiers were on duty in the area when Boko Haram fighters arrived at 11.45pm.

"Only 17 troops were there to face the attack and they were outgunned and outnumbered. They had to flee for their lives together with some other villagers who fled to the bush," Kamara said. "This is a shocking revelation."

The Nigerian military said in a statement: "It has to be categorically stated that the claims by Amnesty International in its report that security forces had advance warning about the abduction of students of Government secondary school in Chibok, Borno State by terrorists is unfounded," it said in a statement.

Ndege, reporting from Abuja, said the Nigerian people would be "extremely shocked and extremely disappointed" to think the Nigerian military knew in advance an attack was going to take place and most would find it "inexcusable".

"It wouldn't make sense to Nigerians. It wouldn't make sense to the parents of the missing girls," our correspondent said.

"The question is, why would the military deliberately choose to ignore this SOS, this cry for help from the community?"

"They say the operation to find these girls is something confidential. you don't give a runnning commentary to the media or the public about something as sensitve as this," she added.

"They are simply not prepared to give the details of any operation to find the girls. They say it might actually harm the girls."