Nigerian security forces were aware that an armed convoy of Boko Haram militants was approaching the town of Chibok almost four hours before the extremists kidnapped 300 girls from a school in the town, Amnesty International said on Friday.

The human rights group claimed the military was warned an attack was imminent but did not send reinforcements because of a lack of resources and an unwillingness to engage with well-armed insurgents. "The fact that Nigerian security forces knew about Boko Haram's impending raid, but failed to take the immediate action needed to stop it, will only amplify the national and international outcry at this horrific crime," said Netsanet Belay, Amnesty International's Africa director.

Shortly before dawn on 15 April, at least 300 girls were kidnapped after sitting their final exams at the Chibok government secondary school, one of the only schools still open in the country's north-east Borno state. Several later escaped, but 276 girls are still believed to be in captivity.

After almost two weeks of public anger at perceived government inaction, American and British anti-terrorism personnel began arriving in the Nigeria capital, Abuja, on Friday to help search for the girls.

Nigeria's president, Goodluck Jonathan, said he believed the victims were still in Nigeria and had not been moved across the border to Cameroon. "There are stories that they have moved them outside of the country. But if they move that number of girls to Cameroon, people will see, so I believe they are still in Nigeria," Jonathan said.

Amnesty based its report on an interview with an army colonel and lieutenant colonel. Their accounts were corroborated with residents and local government officials. "The information we received from them gave us a consistent account and timeline," said Nigeria researcher Mamkid Kamara.

A spokesman for the Nigerian army dismissed Amnesty's report as a "rumours and allegations." General Chris Olukoladu said: "They just want to give a dog a bad name in order to hang it. Their allegations are unfounded as usual."

But Amnesty's report tallies with accounts of the raid given to the Guardian byChibok residents. They say they received phone calls alerting them of the impending attacks. Attempts to alert the local military post by telephone were hampered by unanswered calls and poor phone networks . One group of residents said they dispatched a motorbike rider to the nearest battalion but no reinforcements came.

The dozen or so soldiers posted to Chibok were outnumbered and outgunned by up to 100 militants, witnesses said. The soldiers held off the militants for almost an hour but fled when no reinforcements arrived, according to parents of the missing girls who later encountered the soldiers sheltering in the bush surrounding the town.

"Local vigilante groups passed on the first information to the military at around 7pm. The first gunshots [from the attack] were heard around 11.45pm," Kamara said. The senior military officials passed on the information to their superiors shortly after receiving it, but got no order to send backup to Chibok, half an hour's drive from the nearest battalion.

The head of the national examinations board this week said he had been "pressured" into holding the exam despite worries about security in Chibok. Charles Eguridu said he had received safety assurances from the state governor, Kashim Shettima.

A government official and army general said the state government had not requested additional security in Chibok, although every other school in the area had been razed or attacked by Boko Haram. "It's easy to say the attack was coming with hindsight, but all you have to do is look at a map of the area. How many schools had Boko Haram spared there? None. So why open this one?" the general added.

Local youths, known as the civilian JTF – after the military's special operations squad called the Joint Task Force – have started policing areas where Boko Haram members are known to operate. A senior security source told the Guardian that information from the civilian JTF had helped the military foil at least two earlier attacks in Borno state. But rights groups have expressed worries about the ability of a group without training to act as a law enforcers.

Tangled party politics before the presidential elections, due by February 2015, has come into play following the kidnapping. The opposition controls Borno state – the stronghold of Boko Haram – as well as neighbouring Yobe and Adamawa states. Vast swaths of the three states are under the extremists' control, which prompted President Jonathan to declare a state of emergency in all three last year.

The government has accused members of the opposition of encouraging and funding the attacks; they in turn accuse Jonathan of failing to take seriously the issue of terrorism and the underlying poverty that fuels it. Earlier this week, the president's wife, Patience, publicly suggested the attacks were a fabrication intended to derail her husband's campaign. A further 10 girls were kidnapped in the border village of Waraba this week.

Local residents have said they are angry at being caught in the middle of such political games. "The only place where there is safety now is where the civilian JTF are operating. That means we depend on ourselves," said Kabir Abdulhameed, a resident of Borno's capital, Maiduguri.