Libyan authorities have accused aid groups of plotting to change the country's ethnic make-up by encouraging African migrants to stay there, and has ordered them to close their offices.

Ten groups have been singled out - including Doctors Without Borders, the UN refugee agency, and Norwegian Refugee Council.

"This plan to settle migrants of African origin in our country represents a hostile act. It aims to change the demographic composition of the country and threatens the balance of Libyan society," said Internal Security Authority spokesman Salem Gheit on Thursday.

It echoes a similar announcement made by Tunisia two years ago, which was swiftly condemned as anti-black racism.

Both nations sit on the Mediterranean coast and are key transit points for African migrants crossing the sea to Europe.

Since the overthrow of Libya's leader Muammar Gaddafi in 2011 there has been a breakdown in government, allowing armed militias and human-traffickers to proliferate.

The country has been divided into two, each run by a rival administration.

Militiamen have been accused of running detention centres where migrants are beaten to death or starved, and the Libyan coastguard is accused of sometimes filming people in the sea rather than rescuing them. The Libyan authorities have not commented on these accusations.

Thursday's order to expel the aid groups was made by the internationally recognised government based in the capital, Tripoli.

Doctors Without Borders (MSF) suspended its work in Libya a week ago, describing a campaign of harassment by Libyan authorities "summoning and interrogating the staff of international NGOs" since mid-March.

"Our organisation is very concerned about the consequences that these orders will have on the health of patients and on the safety of humanitarian workers," MSF said in a statement sent to the BBC.

In response to Thursday's announcement, the UN's refugee agency (UNHCR) has defended its work, telling the BBC that the people it helps are not "migrants" but refugees in dire need.

It also says it operates with the consent of the Tripoli government.

"We are in contact with the authorities in Libya and are following up with them to seek clarity. UNHCR has been operating in Libya for over 30 years, providing humanitarian assistance to refugees, asylum-seekers and vulnerable Libyan communities," spokesman William Spindler told the BBC.

One of the accusations reportedly made by Libya's International Security Authority against the 10 aid groups was that they support "illegal migrants by providing them with food, clothes, and medicine, which encouraged these migrants to consider Libya as a final destination and not a transit country". But many say they do not want to stay in the country.

For years sub-Saharan African migrants have been subjected to grave rights abuses and dehumanising treatment in Libya - including being killed, enslaved, or repeatedly raped.

"He used to call me a 'disgusting black'. He raped me and said: 'This is what women were made for,'" a Sudanese refugee trafficked in Libya told the BBC this year, about a man who had offered her a job cleaning his house.

"Even kids here are mean to us, they treat us as beasts and sorcerers, they insult us for being black and African, are they not Africans themselves?"