Boko Haram has been further condemned after details emerged of a massacre of as many as 300 people close to Nigeria's border with Cameroon, while the Islamist group continued to hold more than 200 kidnapped schoolgirls.

The US president, Barack Obama, said the kidnappings and murders in Nigeria, as well as the war in Syria and other conflicts, showed humanity's "darkest impulses".

The latest insurgent attack in Nigeria targeted the town of Gamboru Ngala. Gunmen burned buildings and fired on civilians as they tried to flee. The Nigerian senator Ahmed Zanna put the death toll at 300, citing information provided by locals.

Zanna said the town had been left unguarded because soldiers based there had been redeployed north towards Lake Chad in an effort to rescue the kidnapped girls. Witnesses said Boko Haram fighters riding in armoured trucks and on motorcycles had stormed Gamboru Ngala after midday on Monday and overrun the town.

A medical officer in the Cameroon army, which has reinforced security at the border, said it believed more than 200 people had been killed in the town. "Some of the bodies were charred. It was horrific. People had their throats slit, others were shot," he said.

On Wednesday the British government announced it would send a small group of experts to Nigeria to assist with the hunt for the missing schoolgirls. The team will work alongside US military and law enforcement officers tasked by Obama with providing technical assistance to the Nigerian authorities.

Boko Haram is holding 276 girls after a raid on a school in Chibok on 15 April, and a further eight girls aged between eight and 15 taken in an overnight raid on a village on Monday, also in its stronghold in north-eastern Borno state.

In a video released on Monday, Abubakar Shekau, the leader of the group's main faction, threatened to sell the schoolgirls as slaves.

The US has stopped short of pledging troops, but Britain has said it is prepared to send special forces and intelligence-gathering aircraft. France and China have also offered assistance.

The Nigerian president, Goodluck Jonathan, accepted the UK offer of help in a phone call with David Cameron, shortly after the prime minister told the Commons that the mass abduction was "an act of pure evil".

It is understood that the British team, from Whitehall departments including defence, international development and the Foreign Office, may include military officers but will concentrate on planning, coordination and advice to local authorities, rather than getting involved in operations on the ground to free the girls.

Nigeria's response to the kidnappings has been widely criticised, including by activists and parents of the hostages who say the military's search operation has been inept so far.

The Pakistani schoolgirl Malala Yousafzai, who survived being shot in the head by the Taliban, said on Wednesday she saw the kidnapped schoolgirls as her sisters.

She said Boko Haram did not understand Islam. "They are actually misusing the name of Islam because they have forgotten that the word 'Islam' means 'peace'," she told CNN.

She added: "When I heard about the girls in Nigeria being abducted I felt very sad and I thought that my sisters are in prison and I thought that I should speak up for them."