A deal for Rwanda to host hundreds of African migrants currently being held in Libya has been signed by the Rwandan government, the UN refugee agency (UNHCR) and the African Union (AU).

"A first group of 500 people, predominantly from the Horn of Africa, will be evacuated, including children and youth at risk," a joint statement says.

The evacuation flights for those willing to go to Rwanda were expected to begin in the coming weeks, it added.

Thousands of migrants head to Libya each year to try and make the dangerous crossing over the Mediterranean to reach Europe - those who fail or are caught by the authorities when they arrive end up in detention centres.

According to the joint statement, about 4,700 people are currently estimated to be held in "dire conditions" inside these centres.

Rwanda made the offer to give refuge to migrants in November 2017 after CNN released a video showing men being auctioned off as farm workers in Libya.

The policy of housing migrants within Africa is one way the EU is using to stop African migrants from arriving on its shores.

Options for the migrants would be worked out by the UNHCR once they arrived in Rwanda, the statement said.

"While some may benefit from resettlement to third countries, others will be helped to return to countries where asylum had previously been granted, or to return to their home countries if it is safe to do so.

"Some may be given permission to remain in Rwanda subject to agreement by the competent authorities."

A similar deal exists between the EU and Niger, where migrants from Libya are hosted as they wait to be processed before being granted passage to Europe, Camille Le Coz, from the Migration Policy Institute, told the BBC's Newsday programme.

She agreed little was also known about how the country stood to benefit from the deal, apart from the prestige of demonstrating solidarity with fellow Africans.

The UNHCR says over the last two years it has evacuated more than 4,440 migrants out of Libya, which has been turmoil since long-time leader Muammar Gaddafi was toppled and killed in 2011.