LONDON (USA Today) — Slavery is thriving in the United Kingdom, as authorities try to battle a growing problem.

Britain's National Crime Agency said cases of forced labor affect "every large town and city in the country.”

A government estimate of 13,000 victims in the U.K. is the "tip of the iceberg," according to Britain's anti-slavery commissioner Kevin Hyland. He called for tighter regulation of nail salons in the country — some of which are sites of forced labor.

“This is a really complicated crime," said Rachel Harper, manager of the national Modern Slavery Helpline, a free 24/7 service for victims and those who want to help them report crimes. "It's incredibly intense and traumatic."

Three people who forced Vietnamese women and girls to work at nail bars in different parts of the county were jailed this week under human trafficking and modern slavery legislation.

Thu Huong Nguyen, 49, was sentenced to five years' imprisonment, Viet Hoang Nguyen, 30, got four years, and Giang Huong Tran, 23, was given a two-year suspended sentence.

“The victims worked for no money and were trafficked between nail bars according to demand,” state prosecutor Eran Cutliffe said. “They were hidden from the authorities in order to avoid detection while being exploited in plain sight within our society.”

Police said it was the first successful prosecution for child labor exploitation and child trafficking under the Modern Slavery Act of 2015. The victims include girls under 18 who were smuggled into the country, beaten and verbally abused.

In one well-publicized case that shocked many Britons last year, 11 members of one family were found guilty of forcing vulnerable men with learning disabilities and homeless men to work and live in squalid conditions for up to 16 years.

The National Society for the Prevention of Cruelty to Children said child slaves from countries including Vietnam, Romania, Nigeria and Afghanistan are being forced to work at cannabis farms and commit street crimes in Britain.

Most slaves are trafficked to the U.K. from other countries and forced to work in agriculture, construction, hospitality, manufacturing and car washes. Many women and girls are trafficked for sexual exploitation.

The national Modern Slavery Helpline was set up by Unseen, a British charity that works toward ending slavery globally.

It began taking calls in October 2016. Since then, it has dealt with more than 1,000 cases of modern slavery, said Harper.

Harper said the abuses may have been going on for years but only now have come to light because of better reporting.

“Was all of that happening two years ago and we didn't know, or is it actually happening more often?” she said. “I think it's been more prevalent than we realized and awareness is growing.”

Worldwide, there were more than 40 million victims of slavery in 2016, with women and girls making up 71%, the United Nation's International Labor Organization's (ILO) Global Estimates of Modern Slavery 2017 report said.

In the United States, Polaris, an organization that helps human-trafficking victims, said many workers at restaurants, food trucks and bars lured to the country from other nations are forced into a form of slavery, working long hours, living in cramped arrangements and forced into debt.

A USA TODAY network investigation last year found that hundreds, if not thousands, of truckers are working as indentured servants.

A video that emerged late last year showing west African men apparently being sold into slavery in Libya caused global outrage.

ILO figures show that forced labor in the global economy generates $150 billion in illegal profits a year.

Pope Francis has rallied against such abuses.

"Every year, thousands of men, women and children are innocent victims of labor and sexual exploitation and organ trafficking,” he said. “It seems that we are so accustomed to it that we consider it a normal thing. This is ugly, cruel, criminal! I would like to call on everyone to counter this aberrant plague, a form of modern slavery.”

Harper stressed the importance of collaboration between the Home Office, immigration policy officials, police, service providers, businesses and others. “I do not think that we can prosecute our way out of it," she said.

Detective Inspector Charlotte Tucker, who helped prosecute the nail salon gang, described the case as “desperately sad.”

“As a community, we need to look out for the warning signs and do our part to stop this archaic practice once and for all,” she said.