Nigeria's army has rejected as "unfounded" statements by Amnesty International that it had early warnings of a Boko Haram attack on a Chibok girls' school last month, which led to the abduction of more than 200 pupils.
Amnesty said on Friday it was told that security forces were given four hours' notice of the April 14 attack, but failed to reinforce the town.
Two politicians from Borno state, which borders Chibok, separately told Al Jazeera's Yvonne Ndege that the army had at least two hours' notice.
In a statement on Saturday, the Nigerian army said: "It has to be categorically stated that the claims by Amnesty International ... that security forces had advance warning ... is unfounded."
This is a shocking revelation.
The schoolgirls remain missing and the government's slow response to the abduction has led to protests around the country. Teams from the US, UK and China have arrived in Nigeria to help the search.
Makmid Kamara, a Nigeria researcher for Amnesty, said: "We received information and we spoke to a senior Nigerian military officer ... 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 told Al Jazeera that senior officials in Maiduguri and Dambua towns were alerted at about 7pm on April 14, and that information was given to 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.
"Only 17 troops were there to face the attack and they were outgunned and outnumbered.
|Sources say the Nigerian army was warned about the abductions
"They had to flee for their lives together with some other villagers. This is a shocking revelation."
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".
"The question is, why would the military deliberately choose to ignore this SOS?" said our correspondent.
"They are simply not prepared to give the details of any operation to find the girls. They say it might actually harm the girls."
Meanwhile, specialist teams from the US and the UK in areas that include intelligence gathering, satellite imagery and hostage negotiations began arriving in Nigeria to assist with the search.
Michelle Obama, the wife of the US president, also used the president's weekly address in the US to talk about the abductions, which she described as an "unconscionable act was committed by a terrorist group".
Mrs Obama was recently featured on Twitter holding a card which read "bring back our girls".
On Saturday Nigerian celebrities joined about 100 protesters in a march to the state governor's residence in Lagos to call for the rescue of the girls.