A senior Nigerian official says plans to rescue thousands of victims of sex trafficking have failed.
Investigators went to Mali earlier this year and estimated that at least 20,000 Nigerian teenagers and women had been smuggled there and forced into prostitution.
A joint operation was then launched with Malian authorities to rescue the women.
But Nigerian officials say they have not had enough co-operation from Mali.
The authorities in Mali have refused to comment on the criticism.
Yemisi's story
"I feel like committing suicide when I remember," says Yemisi Ogoda about her journey which ended in Mali.
She was handed to a gang who demanded she repay debts for her travel.
After being beaten and locked up without food for two days, the young Nigerian agreed to work as a prostitute to try to pay the debt.
"They said they would kill me if I did not do it. Nobody will know about it, they will just kill me there, no-one will know," she adds.
She fell pregnant, and was coerced into an abortion. The attempt failed, and, visibly pregnant, she was sold on to another gang, only managing to flee when she was left unsupervised to bathe.
Ms Ogoda survived by begging in the streets until she managed to contact her family.
'Trolley boys'
The network of migration routes that criss-cross West Africa are known to police as a "hot graveyard for migrants", because the number who die on the way is so great.
So-called "trolley-boys" - the trafficking middle-men - run "the relay race", passing their human cargo onwards, with promises of jobs in hairdressing and supermarkets.
The true nature of the "job" is revealed later.
After receiving reports of sexual slavery from aid workers and clergy, Nigerian officials went to Mali to investigate earlier this year.
They said were "nauseated" by what they had seen: Brothels with cubicles in which young Nigerian women, many in their mid-teens, serviced as many as 20 or 30 clients a night, in order to pay off debts incurred to the "trolley-men".
"It is clear it is not consensual," says Arinze Orakwue of Nigeria's National Agency for the Prohibition of Traffic in Persons (Naptip). "They have no freedom of movement. They are not allowed to go outside with you, or even to make a phone call."
Naptip's hard-hitting findings, published on 29 September, also warned of what officials described as "slave camps" in Mali's north - brothels in the gold-mining towns of Kayes and Mopti.
Photographs seen by the BBC reveal precise locations and buildings examined by the team.
'Foreign racket'
The findings were given to Malian police and Operation Timbuktu was launched.
"Operation Timbuktu will be executed with the Malian authorities, to free the girls and ensure their safe return to Nigeria," Simon Chuzi Egede, executive secretary of Naptip, said at the time.
Months later, nothing has happened, and the trafficked teenagers and women remain in the hands of criminal gangs in Mali.
"All of us have failed," says Mr Orakwue, an assistant director of intelligence and communications at Naptip.
"The first thing that is preventing their return is support from the Malian authorities," he says.
"What we want Mali to do is say: 'Nigeria, come! We will support you to strike, to engage in law enforcement action, to get the girls back.'"
Naptip officials say that despite assurances of cooperation from Mali, attempts at communication with the Mali police are being ignored.
It is clear that Operation Timbuktu is beset by difficulties: A lack of French-speakers in Nigeria's police able to communicate with Malian officials, slow bureaucracies, and little political interest in the fate of the victims.
Officials say to make headway, they need wholehearted support from Mali.
"There is a perception that it is a Nigerian problem," says one officer. "These are Nigerian women, controlled by Nigerian gangs. So they see it as a foreign racket. But the customers are in Mali."
Mali is a signatory to the UN Palermo Protocol on people trafficking, as well as the UN Convention on Transnational Organized Crime of 2000.
"Definitely, I want to see diplomatic pressure on Mali," insists Mr Orakwue. "It is an emergency."
Logistical issues
Naptip estimate the number of women trapped in Mali to be anywhere between 20,000 and 40,000. Other estimates - from local charities - suggest a lower figure, in the thousands.
Whatever the number, the idea of raiding brothels and mounting a large-scale repatriation of thousands of people across five West African countries poses serious logistical and financial difficulties for Naptip.
To get started, the agency will need wider support. Officers are impatient to begin work.
"We've got visuals, where these girls are located," says Mr Orakwue.
"And we're here, not doing anything about it," he says. "We're just sitting here, talking."
