A nightclub manager in Amman greets his guests with a warm welcome and, unusually for Jordan, an alcoholic drink. He tells them the girls will arrive shortly.

A man on a microphone introduces the women, who make their way into the club through a cloud of thick cigarette smoke. "Aaliyah" introduces herself, explaining that she is not allowed to sit down, only to stand and dance.

She is wearing a low-cut top and the tattoo on her arm says "life has many obstacles" in Arabic. As the night wears on she tells her story, one of disappointment and exploitation. Aaliyah came to Jordan alone from Syria, fleeing the war but leaving her family behind. She married a Jordanian man who promised to look after her. But she has come to regret her decision.

"I was a virgin before I married, but after three months he got bored and divorced me. I cannot go back home to my family and tell them what has happened, I feel ashamed," she admits. Now she works as an escort and sends some of her earnings to her family in Damascus. "I give them money every month, which helps them, but I do not tell them what I do. They think I am studying."

Aaliyah works with a friend, Ishtar, who brushes off questions, saying only that she has sex with many men. Two days later the Guardian receives a smuggled letter from Ishtar, written in Arabic.

The Zaatari refugee camp is home to more than 100,000 Syrian refugees. Photograph: Jeff J Mitchell/Getty Images

"I left my family and my son in Syria, and I met a man from Palestine who brought me to Amman. In the first three months he treated me well, but then he started forcing me to work in bars and in illegal activities. He made me sign a marriage certificate with certain conditions. My son is sick and I can't go to see him, because this man takes all the money I earn from work. The situation of my family is so miserable. I don't know what to do. I hope you can help and find a solution for me."

Aid workers are warning that young Syrian women in Jordan are increasingly at risk of exploitation, with many struggling to survive in a country where they are not allowed to work. Some have families who are reliant on aid or meagre savings.

Amira Mohamed is a counter-trafficking officer at the International Organisation of Migration (IOM) in Amman. She is preparing for the next stage of an awareness campaign aimed at Syrian and Jordanian families. "We have had lots of reports from the media and people on the ground that trafficking is becoming a problem, this typically happens where there is conflict and [there are] vulnerable people," she says.

The project highlights how Syrian women are vulnerable to being trafficked through forced marriage and sexual exploitation, and children are pushed into work. "[Syrian families] might not realise they are being exploited, so we try to prevent this by organising projects, for instance ... we get the Jordanians and Syrians working together, cleaning the streets and giving out aid parcels. Within the [aid] boxes there is a message explaining the signs of human trafficking."

She points to the difference in dowries being paid for brides as one of the ways which poverty leads to exploitation. "For example, culturally, early marriages happen in Syria [where] they might get $1,000 – but as refugees in Jordan they might get $100."

Marriages of under-18s being conducted by imams or sheiks in the camp are no longer recognised by Jordanian courts. Photograph: Lee Harper for the Guardian

When the Guardian shows her the letter from Ishtar, she says it is the type of evidence they are hoping to see more of as their project continues. "This is a perfect example of trafficking. She is asking for help and I must follow it up and make a case to the public security department unit, [which is] combating human trafficking."

The Zaatari camp is home to more than 100,000 Syrian refugees. An olive-green tent standing apart from the others is used as a mosque. The imam, speaking on condition of anonymity, admits he worries about some of the young women he sees. He says he is so concerned that he refuses to carry out some marriages.

"The men come into the camp and … they are just buying girls." He says many of these men come from Jordan and the Gulf; most are seeking approval to wed a much younger woman. "When I was in Syria, I used to sign the marriage papers, but here ... only when they are over 18. I am against the marriages in the camp, unless it is registered by the Jordanian government."

Jordan enacted an anti-trafficking law in 2009, and has also ratified UN conventions against offence. Jordanian law has been applied to the camp, and marriages of under-18s being conducted by imams or sheiks are not recognised by the courts. The Zaatari camp commander, Zaher Abu-Shahab, a Jordanian colonel, says: "If I see one of these marriage agreements, I tear them up and throw them in the bin in front of them."

The head of security, Colonel Eid al-Qarara'a, denies there are forced marriages, or that men are coming into the camp to look for vulnerable young women. "Syrian society is very conservative, and until now we haven't [seen] any sort of event like this," he says.

Colonel Eid al-Qarara’a, head of security at the Zaatari camp, denies forced marriages are taking place. Photograph: Lee Harper for the Guardian

Kilian Kleinschmidt, manager of the Zaatari camp, told the Guardian the UN refugee agency had made gains in tackling the exploitation of young women through tighter controls on the camp and outreach work. But he added that it was just as important to push for protection for couples who marry legitimately within the camps.

"We are working together with the Jordanian authorities to verify marriages because unless they are recognised by the Jordanian court system, this leaves women with no legal protection if the husband runs away," Kleinschmidt said. "Marriages have to be countersigned by the court. Couples are leaving the camp to go elsewhere in Jordan, or even back to Syria, and they are finding their marriages are not recognised."

Just six miles down the road from the camp is the town of Mafraq, home to more than 65,000 registered Syrian refugees. They receive far less support than their compatriots in the camps. Mafraq is a key focus of the IOM campaign to raise awareness about trafficking.

Tamira lives in Mafraq with her husband and 16-year-old daughter. Battool attends school but her youth has not protected her from propositions of marriage. Tamira says she has had to warn off a Jordanian man and an Iraqi.

"A friend of my husband's friend came to the house and asked if I wanted to help them to find Syrian girls for marriage. They wanted tall, blonde and thin girls. My daughter is brunette, so they didn't take her," she laughs. The two men said they would give Tamira a percentage of the fee they were getting from their Saudi contacts.

"They regularly go house to house looking for brides in Mafraq. In the end they went to a matchmaker and found a girl," Tamira says. "They come to Jordan to find Syrian girls because they think we are needy people now."

Names have been changed to protect identities