Britain is to send a small group of experts to Nigeria to help with the hunt for almost 300 schoolgirls kidnapped by Islamist extremist group Boko Haram, Downing Street has announced.

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

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

Cameron's official spokesman said the UK team will fly to the west African state "as soon as possible", but was unable to say how large the group will be.

The British team will work alongside US military and law enforcement officers tasked by President Barack Obama to provide technical assistance to the Nigerian authorities. Downing Street stressed that Nigeria's government remains in the lead in the operation to find the 276 missing girls.

Talks are under way through the British embassy in Washington and the high commission in Nigerian capital Abuja to establish "in short order" what assistance the UK team can most usefully offer.

The leader of Boko Haram – whose name means "western education is wrong" – has threatened to sell the girls abducted three weeks ago into slavery.

At prime minister's questions in the House of Commons, Cameron said it was vital that Britain stood against the actions of "extreme Islamists" who oppose education and progress.

He said: "I'm the father of two young daughters and my reaction is the same as every father or mother in this land or the world.

"This is an act of pure evil. It has united people across the planet to stand with Nigeria to help find these children and return them to their parents.

"The foreign secretary and the British government have made repeated offers of help to the Nigerian government since the girls were seized. I'll be speaking to the Nigerian president this afternoon and will again say Britain stands ready to provide any assistance immediately we can, working very closely with the US."

Cameron's official spokesman said that in a telephone call shortly afterwards, Cameron assured Jonathan of his "shock at the appalling attack and his whole-hearted support for the Nigerian people during this very difficult time".

The spokesman said: "The prime minister reiterated the UK's offer of assistance to the Nigerian authorities. President Jonathan accepted the prime minister's offer to send a small team of experts drawn from across Whitehall departments to complement the US team committed by President Obama."

Cameron's spokesman rejected suggestions that the UK had been slow to provide assistance following the abduction on 15 April.

"We have been in close contact from the outset with the Nigerian government on this issue," said the spokesman.

"The foreign secretary spoke to his Nigerian counterpart days after the incident in mid-April. We have been in regular contact through the Foreign Office and high commission at all stages, making clear that an offer of assistance that the Nigerian government would find helpful was there."

Downing Street said Cameron and Jonathan agreed to "explore how to further strengthen co-operation on counter-terrorism in the longer term, so as to help in the prevention of such attacks in the future".

Cameron's spokesman added: "The prime minister and president very much agreed about the importance of the international community continuing to work together to meet the challenge posed by Islamist extremism and violence."

Britain already has a small military training team in Nigeria, assisting the country in its response to the extremist groups behind a five-year Islamist uprising which has killed thousands.

The announcement of the new assistance came as reports emerged of an attack by Islamist militants on the north-eastern border town of Gamboru Ngala, which cost the lives of as many as 300 people.

Cameron told the Commons: "We should be clear this is not just a Nigerian issue – it is a global issue. There are extreme Islamists around our world who are against education, against progress, against equality and we must fight them and take them on wherever they are."

Obama has said the US will do "everything we can" to help Nigeria find the missing girls, adding: "We're also going to have to deal with the broader problem of organisations like this that … can cause such havoc in people's day-to-day lives."

The US team will include military and law enforcement personnel skilled in intelligence, investigations, hostage negotiation, information-sharing and victim assistance, as well as officials with expertise in other areas, said the White House. But spokesman Jay Carney said the US was not considering sending armed forces.