Khasim is trapped in Greece. Two years ago, the Balkan countries to the north of Greece closed their borders, blocking the migrant route to Western Europe. The United Nations estimates that some 50,000 asylum-seekers and refugees remain in the country.

Greece has the highest unemployment rate in Europe — more than 20 percent — and is still grappling with the fallout of the 2008 debt crisis, which left an already-stressed welfare system struggling to cope.

Khasim is among the unaccompanied minors who have resorted to prostitution to meet their basic needs — a practice known as "survival sex."

While there is no official data as to how many young asylum-seekers engage in survival sex in Greece, a report by Harvard-based academics published last year found that the victims tend to be young males. The majority come from Afghanistan, while Syrians, Iraqis and Pakistanis are also resorting to prostitution to survive, according to local social workers.

'My family think I’m staying in the best hotel in Athens'

For Khasim, the aim was always to reach Germany. His hometown, Kabul, was torn apart by regular bombings and there were few job prospects and no hope, he said.

He trekked across Iran, Turkey, Bulgaria and Serbia only to find his path blocked by the fortified border with Hungary. After months on the road, Khasim says he spent a year living in Belgrade and looks back at his time in the Serbian capital fondly, showing a picture of himself on the banks of the Danube River. Khasim eventually headed south to try his luck in Greece.

Khasim's epic journey to Greece started in Kabul, Afghanistan. NBC News

When NBC News met Khasim last month, he had already tried to leave the country four times using stolen or fake passports, but kept getting caught.

"Athens is so bad," he said via Facebook Messenger after his most recent attempt failed. "If I do not go to Germany, I'll kill myself."

As he waits to obtain another illegitimate passport, the teenager walks the streets with his friend Fazul, who also asked to be identified only by a pseudonym.

Fazul, who is from Afghanistan and says he was living in Iran when he set off for Europe, claims to be in his early 20s. However, Khasim thinks he is younger. "He’s a baby, he’s underage. He says he’s 22, but he’s 17 or 16. Look at his face," he said in English, so Fazul could not understand.

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Khasim says he used to earn as much as 40 euros ($50) for turning a trick but an increase in the number of young, desperate migrants from countries including Pakistan and Iraq have sent prices plummeting.

"Now it’s more like 10 euros ($12), 5 euros ($6)" for some sex acts, according to Khasim.

Fazul admits that he has agreed not to wear a condom on several occasions, but says that only gets him 5 euros extra.

Khasim says he's refused to have unprotected sex. "His mind’s not right," he said, referring to Fazul's risky behavior.

Once the young migrants connect with customers in Omonia Square, they often exchange cellphone numbers to organize meetings via text message. But Khasim and Fazul also still work on the streets.