US First Lady Michelle Obama has spoken of her outrage and heartbreak at the abduction of over 200 Nigerian schoolgirls by Islamist militant group Boko Haram.

Taking the place of US President Barack Obama on the weekly radio and internet presidential address, Mrs Obama described the mass abductions as an "unconscionable act" by grown men attempting to "snuff out the aspirations of young girls".

"What happened in Nigeria was not an isolated incident," the First Lady said. "It's a story we see every day as girls around the world risk their lives to pursue their ambitions."

"In these girls, Barack and I see our own daughters," Mrs Obama added, in reference to Malia, 15, and Sasha, 12. "We see their hopes, their dreams and we can only imagine the anguish their parents are feeling right now."

Speaking ahead of Mother's Day in the US on Sunday, Mrs Obama also made reference to Pakistani schoolgirl and activist Malala Yousafzai, who was shot in the head by the Taliban in 2012 for campaigning for the right of girls to be educated.

"The courage and hope embodied by Malala and girls like her around the world should serve as a call to action," the First Lady said.

Michelle Obama has often appeared alongside her husband in previous weekly addresses but this is the first time she has delivered a speech on her own.

Earlier this week, Mrs Obama tweeted a picture of herself in the White House holding a sign featuring the hashtag #BringBackOurGirls.

The hashtag, which was created by Nigerian lawyer Ibrahim Musa Abdullahi, has been retweeted nearly 2 million times. A series of high-profile names, including former US Secretary of State Hillary Clinton, have also given their support to the social media campaign.

At least 276 girls were kidnapped from their school by Boko Haram in the north-eastern Nigerian village of Chibok on 14 April. While 53 victims managed to escape, at least 223 are still missing.

A series of protests expressing anger at the abduction have taken place around the world, and the UN Security Council said it would consider "appropriate measures" against Boko Haram. The US is also seeking to have UN sanctions imposed on the group.

World powers including the UK, US, France and China have offered assistance to the Nigerian government in the search for the missing girls.

The US is reportedly sending FBI agents to join a US military team in the search, while UK Special Air Service will assist Nigerian officers in attempting to intercept voice communications across large areas of the Nigerian jungle alongside.

Canada also recently offered to provide the Nigerian government with surveillance equipment and technical expertise in the hunt for the missing schoolgirls.

Watch Michelle Obama's address below: