Britain has offered assistance to Nigeria to help respond to the kidnap of about 300 schoolgirls by Islamist militants, Foreign Secretary William Hague has said.

His comments came after the leader of Boko Haram, Abubakar Shekau claimed responsibility for the April 15 abduction and threatened to sell the girls into slavery.

In a video message, a gun-toting Shekau said: "They are slaves and I will sell them because I have the market to sell them."

Mr Hague declined to give details of the help which Britain may be able to offer the Nigerian authorities, and stressed that the principle responsibility for dealing with the incident rests with the government in Abuja.

But he denounced the kidnapping as "disgusting (and) immoral" and called on all countries to withhold support from the "vile" Boko Haram.

Protests have been staged in Nigerian cities over the failure of the administration of president Goodluck Jonathan to take action to return the schoolgirls to their families. Nigerian police say that some 276 girls remain in captivity, while 53 have escaped.

Speaking ahead of a meeting of the Council of Europe in Vienna, Mr Hague said: " It is disgusting, it is immoral, it should show everybody across the world that they should not give any support to such a vile organisation.

"Our hearts go out to (the girls') families. I called the Nigerian foreign minister when this first arose back on Good Friday, in the middle of last month, to offer help from Britain, to express our concern."

Asked how Britain might be able to assist the Nigerian authorities, Mr Hague said: "I don't think it's possible to go into the details of precisely what help we could provide, but we have offered assistance, our High Commissioner in Nigeria continues to offer assistance. We continue to discuss that with the Nigerians.

"Britain is offering assistance, but of course the primary responsibility for dealing with this rests with the Nigerians and we hope they will do what is necessary to reunite these girls with their families."

Mr Hague will host a summit in London next month of the Preventing Sexual Violence Initiative, which he launched in 2012 in response to the issue of mistreatment of women and girls in conflict areas.

"This is to help countries, including Nigeria, improve the way they tackle the consequences of sexual violence and make sure that those responsible can be brought to justice, that the evidence is gathered, that those who are victims of it can be properly looked after in the future," he said.

"Recently we've persuaded Nigeria to support that campaign, so this in the future may help. Of course, it doesn't help today with the situation of these girls."