Three men and 12 women, who were potential trafficking victims, were rescued from properties across the UK in the raids on Friday.

A total of 16 properties were raided - of these 13 were exposed as brothels and seven were in Belfast.

Four people have been arrested, some of whom outside of Northern Ireland, and are being brought to the country for questioning.

It was part of the UK-wide 'Operation Apsis', which police say was aimed at recovering victims, both UK and foreign nationals, who are subject of trafficking for the purpose of sexual exploitation.

'Slavery'

Detective Chief Superintendent McComb, Head of the PSNI Organised Crime Branch described the horror of human trafficking, saying: "These are human beings, treated as if they were battery hens in a sexual environment.

"They are being robbed of their liberty, stripped of their dignity, and suffer intolerable conditions as unwilling emblems of the sex trade.

"This is modern day slavery where human beings are treated like commodities by crime gangs who are making substantial criminal profits from the sex trade.

"These gangs have no thought for the health and wellbeing of their victims. They see them simply as instruments to help them generate cash."

He said the brothels were being run by sophisticated criminal organisations.

"The movement from on-street prostitution to off-street prostitution - the running of brothels - is a development that has been brought about by the involvement of organised crime," he said.

"What we are seeing today is the hand of organised crime, who has spent considerable time as well as human capital and financial capital to bring about a very sophisticated organised crime network."

The Chief Superintendent warned that anyone involved in supporting the illegal sex trade is funding criminal groups.

He said: "We appeal to those men that are buying sex to recognise that they may be part of the problem, that they may be continuing the exploitation of vulnerable men and women that they may be breaking the law, but that they are supporting and funding organised crime."

South Belfast Alliance MLA Anna Lo said she was not surprised by the discovery.

"We do know there is a problem of trafficking and we believe it is on the increase, not just in Belfast but all over Northern Ireland and across the UK," she told UTV.

"Belfast is a transit route for people coming from Dublin to go over the UK and vice versa, so Belfast is particularly vulnerable."

Justice minister David Ford has welcomed the police operation.

He said the potential victims recovered in Northern Ireland will receive the care and support they need "under the DoJ victim support package to help them recover from their traumatic experiences and to help them to rebuild their lives."

"Human trafficking is a vile crime and has no place in any decent society. I am horrified that these victims have been subjected to such treatment," he added.

The PSNI say it is continuing to work with law enforcement organisations around the world to dismantle illegal trafficking operations.

Officers have asked anyone with information to come forward.