Both the US and UK have offered help to secure the release of a group of kidnapped schoolgirls, as the crisis over their abduction by Islamic extremists in Nigeria deepened.

This morning, foreign secretary William Hague, told reporters the actions of Boko Haram, who kidnapped the girls from their school three weeks ago, were “disgusting” and “immoral”.

Mr Hague, who was in Vienna to attend a meeting about the crisis in Ukraine, added that Britain had offered help to the Nigerian government but could not discuss details.

White House spokesman Jay Carney told a press briefing yesterday that it viewed the kidnapping as “an outrage and terrible tragedy”.

He added the US state department has been in regular touch with the Nigerian government about what it can do to support its efforts to find and free the young women.

On Sunday, Nigerian president Goodluck Jonathan pledged to find the more than 200 Nigerian schoolgirls. “We promise that anywhere the girls are, we will surely get them out,” he said. It was the first time he had spoken on the issue since their disappearance.

But yesterday, a man claiming to be the group’s leader Abubakar Shekau, announced plans to ‘sell’ the girls.

In a video obtained by Agence France-Presse, he said: “I abducted your girls. I will sell them in the market, by Allah.”

The threat has fuelled the mounting anger in Nigeria - and around the world - at the failure to find the girls. The twitter hashtag #bringbackourgirls has brought worldwide attention to their plight.

Writing in the online magazine The Conversation, Kyari Mohammed, chair of the Centre for Peace and Security Studies at Modibbo Adama University of Technology, said many teachers and students in the north-east of the country were living in fear since Boko Haram became active in 2009.

He said: “The world is waking up to Boko Haram. More than 200 schoolgirls kidnapped from the classes last month remain missing. A car bomb in Abuja on May 1 killed at least 19 people…Following Boko Haram leader Abubakar Shekau’s reiteration that all schools are targets, we are all living in fear.”

Boko Haram has carried out dozens of attacks on schools - the kidnap came weeks after a boarding school was burnt down as the students slept killing an estimated 40 students.

Some 30 teachers were shot dead in Nigeria from January to September 2013, sometimes during class according to the report Education Under Attack 2014.

The global study of attacks on students and teachers carried out for political, sectarian or religious reasons reported that in Nigeria, Boko Haram had carried out a spate of kidnappings of students and staff for ransom between 2009 and 2011.

But activities escalated in 2011 with increasing reports of killings, burned down classrooms and bombings, with universities targeted as well as schools.