More than 20,000 migrants, including pregnant women and babies, are being held either in detention centres or by traffickers in Libya's people-smuggling hub Sabratha, the UN said Tuesday, warning of abuse "on a shocking scale".

The UN refugee agency said Libyan authorities were holding more than 14,500 migrants who had previously been kept captive by smugglers in and around Sabratha, to the west of Tripoli.

The migrants were discovered in farms, houses and warehouses in and around the coastal city after a force allied with Libya's UN-backed unity government drove out a rival militia earlier this month.

They have been taken to a hangar in the city and are gradually being transferred to official detention centres where aid organisations are providing assistance, UNHCR spokesman Andrej Mahecic said.

Authorities also estimate that another 6,000 people are still being held by smugglers, bringing the total number held to around 20,500, Mahecic told reporters in Geneva.

Following the ouster and killing of long-time dictator Moamer Kadhafi in a NATO-backed 2011 revolution, Sabratha became a major hub for migrants seeking a chance to head to Europe.

Taking advantage of a security vacuum, some local smugglers took control of whole sections of the city and even built their own landing piers, equipped to launch dozens of migrant boats a day.

- Sexual abuse, forced labour -

Mahecic said UNHCR staff working on the ground with those previously held by the people smugglers had described a "picture of human suffering and abuse on a shocking scale."

"Amongst the refugees and migrants who suffered abuse at the hands of smugglers, there are pregnant women and new-born babies," he said, adding that hundreds of people were found without clothes or shoes while hundreds said they had not eaten for days when they were found.

Many of them require urgent medical care, Mahecic said, adding that some had suffered bullet wounds while most said they had been subject to abuse, including sexual violence and forced labour.

UNHCR also warned there was "a worrying number of unaccompanied and separated children, many under the age of six."

The agency said the official detention centres and assembly points were overflowing, and often lacked water tanks and sanitation facilities. 

Many people, including children, are being forced to sleep out in the open.

While smugglers had committed the worst abuse against the migrants, the UN has previously warned of "extremely bad" conditions in Libyan detention centres.

Until recently, Libya has been a major launching off point for migrants, most of them from African countries, trying to cross the Mediterranean to Europe.

But the number of attempted crossings has dropped off dramatically, since Libya's coastguard received European Union funding and training to stop smugglers taking migrants to the water.