Michella Obama and Malala Yousafzai are among a growing chorus of voices who have joined the #BringBackOurGirls viral campaign to free the 284 school girls kidnapped in Nigeria by an Islamic terror group last month.

The public figures, along with celebrities such as Naomi Campbell and Bradley Cooper, have posted photos of themselves holding #BringBackOurGirls signs on Instagram and Twitter.

Pressure from the international community to safely locate the missing school girls - taken at gunpoint from their school in northern Nigeria on April 14 - has ratcheted up in the past few days. Both the British and US government have sent specialist officers and technical assistance teams to the country, in a bid to intensify the search.

Malala Yousafzai has been a vocal campaigner from the outset of the crisis

The girls, aged 16 to 18 years old and from mostly Christian families, are believed to be somewhere in the vast and lawless Sembisa Forest region of northeast Nigeria under the control of radical Islamist group Boko Haram.

There are fears that some of the girls have already been smuggled over the border into Cameroon and Chad, after Boko Haram leader Abubakar Shekau claimed responsibility for their abduction and vowed to sell them as slaves.

"I will sell them in the market, by Allah. I will sell them off and marry them off. There is a market for selling humans," he taunted, in a video obtained by newswire AFP.

The school girls were kidnapped at gunpoint from their dormitories in the middle of the night on April 14

The Nigerian government has faced increasing criticism over its lack of action in the crisis, with military officials struggling to get a hold of the situation.

Yesterday, police officials offered a 50m naira (£177,000 or $300,000) reward on information leading to the location of the kidnapped group.

A statement from the Nigerian police high command promised the money to anyone who "volunteers credible information that will lead to the location and rescue of the female students."

A host of Hollywood men, including Sean Penn, Ashton Kutcher, Justin Timberlake and Bradley Cooper, add their voices to the campaign to free the girls

As well as the 284 girls held from the attack on 14 April, Boko Harem has also abducted a further eight girls, aged between eight and 15, in an overnight raid on a village on Monday.

This brings the total held captive to 284, with a further 50 girls managing to escape as the armed rebels made their way through villages away from the school in Chibok, killing people and burning down buildings en-route.

As the days go by with no sign of their daughters, the girls' family members are becoming increasingly fraught.

"They’d assured us they would rescue our children but... we still haven't seen our daughters," one of the victim's mothers, Rahila Bitrus, told Channel 4. "We are going through the very worst moment of our lives."

"There is no way to describe what we are feeling. It is just one long nightmare," one woman whose daughter and niece were abducted told the Guardian.

Jessica Biel pledges to #BringBackOurGirls

British Prime Minister David Cameron described the mass kidnappings as "an act of pure evil" in an address to Parliament, and pledged special forces and intelligence-gathering aircraft to Nigeria, in an effort to track down the group.

President Barack Obama said the US government was doing its utmost to help resolve a "terrible situation".

"In the short term our goal is obviously to help the international community, and the Nigerian government, as a team to do everything we can to recover these young ladies," he said in an interview with NBC.

Naomi Campbell tweets her support for the girls' safe return

Other public figures have also voiced their dismay over the situation. UNHCR special envoy Angelina Jolie described the abductions as an act of "unthinkable cruelty and evil".

US First Lady Michelle Obama posted a sign on Twitter, with the caption, "our prayers are with the missing Nigerian girls and their families. It's time to #BringBackOurGirls."

Education activist Malala Yousafzai, who survived being shot in the head by the Taliban in Pakistan last year, referred to the girls as "our sisters" as she plead for their release.

Yousafzai also said Boko Haram were "using and abusing the name of Islam."

"In Islam, it is said by the Prophet that education is not only your right, it's your duty to go and get knowledge," she said, in an interview with Radio 4's Today programme. "It's the right of every girl and every boy, so there is no discrimination in Islam."

Michelle Obama urges an end to the mass abduction

Boko Haram's terror campaign in Nigeria has resulted in the loss of over 4000 lives in the past four years. At least 75 people were killed in a car bombing in the capital, Abuja, last week, and a further 50 pupils were massacred at a school in Yobe state in February. It's also thought to be behind a massacre of 300 people in a market in Gamboru Ngala, a town close to the border with Cameroon in Nigeria's lawless northeast region, earlier this week.

The group claims to be fighting for an Islamic state in northern Nigeria, with strict adherence to Sharia law.

Words: Anna Brech, Photos: Twitter and Rex Features