BOGOTA:  It is barely lunchtime on a weekday but in the popular brothels in the Colombian capital's gritty Santa Fe neighbourhood, where scores of Venezuelan women sell sex, the lap dances are in full swing and the shots of alcohol are flowing.

In one brothel in Bogota's downtown red light district, Maria, a former university student from Venezuela, stands beside a pole dancing area with coloured fluorescent lighting, waiting for the first of up to 10 clients of the day.

"I didn't do this in Venezuela. I never ever imagined I'd be doing this in Colombia," said Maria, who declined to give her real name, wearing a skimpy dress and shiny stilettos.

As a humanitarian and political crisis in neighbouring Venezuela deepens, Maria is one of growing numbers of Venezuelan women working in bars and brothels across Colombia.

There are around 4,500 Venezuelan sex workers in Colombia, some working in the capital, others in Caribbean tourist resorts and even in far-flung Amazon villages near the Brazilian border, according to ASMUBULI, a Colombian sex workers association.

Campaigners and the United Nations say Venezuelan migrant women and men selling sex in Colombia are at high risk of being trafficked into forced prostitution but little is known about the true scale of the largely invisible problem.

Venezuelan migrants who have been trafficked into the sex trade are often lured by false promises of well-paid work in Colombia's restaurants and bars or as domestic workers.

But then they find they are forced to work long hours with little or no pay, are not free to leave the bar they work in, and may be trapped by debts owed to the agents who brought them across the border.

Maria, 26, says she had no choice but to resort to prostitution, putting dreams of being a television host on hold and leaving behind her young children, husband and sick mother.

She charges $17 for 15-minutes of sex, and the money earned is spent on buying medicine for her mother who has cancer.

For the past year, she has travelled back and forth from Bogota to Venezuela's capital Caracas every 90 days, before her tourist visa expires, carrying medicine, food and soap.

"I'm ashamed I have to do this. It's a secret," said Maria, who has told her family she is a travelling salesperson.

"But it's the only way to survive and get the medicine my mother needs. Leaving my children is the hardest part," said Maria, as tears welled up in her eyes.

Outside the brothel, skimpily clad women stand on graffiti-strewn corners, while waiters dressed in pink waistcoats snort cocaine and others usher in more customers.

ESCAPING HUNGER

Tens of thousands of Venezuelans have crossed into Colombia in the past year, as triple-digit inflation, a collapsed health system and weeks of violent protests engulfs oil-rich Venezuela.

As prostitution is legal in Colombia it makes it difficult for society to see sex workers as victims of trafficking, and a blurry line often exists between those who voluntarily engage in adult prostitution and those coerced into sex work.

Fidelia Suarez, head of the ASMUBULI sex workers association, says many Venezuelan women are being forced into prostitution in Colombia.

She cited the case of 11 Venezuelan women trapped in a dingy bar in Colombia's northern city of Bucaramanga.

At first, the women were allowed to come and go as they pleased, travelling to and fro between Colombia and Venezuela.

But in February, the bar owner seized their documents, withheld their wages, and prevented them from leaving the bar, Suarez said.

"That's slavery," said Suarez, who has visited the bar. "They are enslaved there, under the conditions and rules decided by the owner which aren't legal."

Thousands of Venezuelans are also spilling over the jungle border Venezuela shares with Brazil every day.

At Boa Vista, the capital of Brazil's border state of Roraima, young Venezuelan women have been forced into prostitution, government officials told Reuters in May.

EASY BAIT

Desperate to send money back home to feed their families, migrants make easy prey for traffickers, campaigners say.

"The situation in Venezuela has made people vulnerable .. they are bait for people seeking to get rich off trafficking women into forced labour," said Carmen Martinez, regional legal director at rights group Women's Link Worldwide.

"Traffickers tell women, 'If you stop what you're doing, you won't be able to send money for your children and they will die of hunger,' she said.

Fear of being deported and threats from bar owners of being handed over to the police often stops undocumented migrant women from speaking out about their own or other trafficking cases.

The Colombian authorities are also failing to identify potential victims of trafficking before deporting women who have overstayed their visas back to Venezuela, Martinez said.

This means the chances of trafficked victims being detected and getting help are almost non-existent, campaigners say.

The increasing number of Venezuelan sex workers in Colombia and the plight they face has caught the attention of the country's constitutional court.

In April the court ruled sex workers should be given work visas and their rights protected.

In an email to the Thomson Reuters Foundation, Colombia's interior ministry, tasked with combating human trafficking, said there were "concerns" that Venezuelan women could become victims of trafficking.

Yet so far this year, the ministry has received just one suspected case of a Venezuelan woman trafficked into the sex trade, while last year three cases of forced labour involving Venezuelan men in Colombia were reported.

MALE VICTIMS

It's not just women who say they have no option but to sell their bodies for sex, but young Venezuelan men too.

A short drive from Santa Fe, 25-year-old Dorian from Venezuela is also looking to get his first client of the day.

Standing against a tree in Bogota's Lourdes Park, under the imposing shadow of the square's gothic cathedral, he and seven other men are also casualties of the crisis in Venezuela.

Without a job and with no money to pay for rent and food, Dorian started working in the square about two weeks ago.

"It's disappointing. I'm disappointed in myself," said Dorian, a business studies university graduate.

Dorian says he left Venezuela six months ago, after a close friend was shot dead by gang members on his way home from a party.

"It could have been me. I knew then that I had to flee, that I was in danger," said Dorian, dressed in tight white trousers.

"The economic instability, the insecurity in Venezuela, it all becomes unbearable."