Libyan authorities deported more than 150 Nigerian women and children on Tuesday under a UN-affiliated "voluntary return" scheme for irregular migrants, an immigration official and UN sources said.

Libya is a key departure point on North Africa's Mediterranean coast for migrants, mainly from other parts of Africa, risking dangerous sea voyages in the hope of reaching Europe.

Mohamad Baredaa of Libya's migration agency told AFP that the migrants deported on Tuesday were all Nigerian "women accompanied by children."

Sources at the International Organization for Migration (IOM), the UN agency involved in the repatriation scheme, said the group included 160 women and 17 children.

More repatriation flights planned

Dressed mostly in black tracksuits, they gathered at a waiting room in a Tripoli detention centre before being taken by bus to the Libyan capital's Mitiga airport.

Baredaa said several more repatriation flights were due to leave this week from Mitiga and an airport in Benghazi, in the country's east, carrying groups of Bangladeshi, The Gambian and Malian migrants.

Violence and instability in Libya since the 2011 overthrow and killing of ruler Muammar Gaddafi in a NATO-backed uprising helped turn the country into a fertile ground for human traffickers.

Smugglers and traffickers have long been accused of abuses.

Libya says it will 'not become a settlement zone'

According to the IOM, there are more than 700,000 migrants in Libya.

Libyan authorities, however, say the actual figure is much higher.

Imad Trabelsi, the interior minister of Libya's UN-recognised government in Tripoli, said this week that there might be "more than four million migrants" in the country, but admitted exact figures were unavailable as many were undocumented.

Seeking to assuage concerns among Libyans, Trabelsi said on Monday that the country "will not bear the burden of illegal immigration alone and will not become a settlement zone."