DAMASCUS — The illegal trade of human organs has become widespread in Syria and neighboring countries, medical officials and victims say, with cross-border networks exploiting thousands of desperate Syrians.

These networks allegedly purchase transplantable organs such as kidneys and corneas from Syrians and ship them to neighboring countries, where they disappear into the murky world of the international organ trade. There are also reports of organs being stolen from prisoners.

Yasser (not his real name) sold one of his own kidneys, which he calls the "worst decision of my life." The 29-year-old fled the fighting in his home city of Homs, in western Syria, around 100 miles north of the capital Damascus, after the start of the war. He made his way to Cairo, Egypt, but like many other Syrian refugees, he had trouble getting work and found himself with no money to survive.

He heard through acquaintances that there were people who would pay for one of his kidneys.

"I was new to Egypt. I did not have any money, and I couldn't find a job, so my only choice was to sell my left kidney," he said.

A broker invited him to his home and a date was set for medical tests and the operation.

"I sold it for $3,000 to someone I knew nothing about. We met for no more than 15 minutes before we closed the deal," he said.

After the operation, Yasser moved to Istanbul, where he now works in an auto shop and shares a crowded apartment with several other young refugee men. The operation has left him permanently scarred — both physically and emotionally — and he felt uncomfortable sharing further details of the procedure.

"I will never forgive myself for what I did," said Yasser. He's experienced pain in his remaining right kidney, and a doctor told him he could die if he is not very careful.

There are no reliable statistics on how widespread the practice of buying and selling organs may be. However, Hussein Nofal, head of the department of forensic medicine at Damascus University and of the newly established General Authority for Forensic Medicine, has been compiling evidence. He estimates that 18,000 Syrians have had organs removed for sale over the past four years of war.

The trade is particularly active in border areas outside the control of Bashar al-Assad's regime and inside Turkish and Lebanese camps for Syrian refugees, according to Nafal. Organ prices vary across the region: In Turkey someone can purchase a kidney for $10,000, while in Iraq the price may be as low as $1,000. In Lebanon and Syria, the cost hovers around $3,000.

Nafal was also quoted last year in the Lebanese newspaper As-Safir, which is reportedly sympathetic to Assad's regime, as saying that gangs working with Syrian doctors sell corneas for $7,500 each to foreign clients, falsifying the organs' country of origin.

Even war-torn countries have laws, but the laws surrounding the organ trade in Syria are opaque, and, with the raging conflict, difficult to enforce or prosecute.

All across Damascus, for instance, hundreds of posters request organ "donations," especially next to hospitals and pharmacies. A typical one reads: "A sick person is in urgent need of a kidney. Blood type needed: O+. Tissue analysis to be done. For those interested in donating, please contact the number below."

Authorities can do little about such advertisements, since under Syrian law organ donations to relatives and strangers are legal. To further evade the law, many organ "donors" who respond to these fliers go to their local courthouse and attest that they are donating, not selling, their organs.

Nevertheless, at least 20 complaints related to the organ trade made their way to the Damascus courts between March 2011 and September 2015. No such cases were seen before the fighting broke out, according to the attorney general of rural Damascus, Ahmad al-Sayyed.

These complaints, which name alleged criminals, as well as doctors and hospitals, have largely been filed by relatives of people who have died. Organ trade cases are considered difficult, if not impossible, to prosecute, since those involved are hard to track down amid the conflict.

However, al-Sayyed estimates that there have been at least 20,000 cases of illegal organ sales across the country since the start of the war, especially in border areas where there are no longer any courts or police officers to enforce the laws.

A judicial source at the Syrian Ministry of Justice who asked to not be named said police do not have the resources to follow up on individual cases to ensure that a person receiving an organ has not paid the "donor."

One oncologist, Dr. Mohammed Awram (not his real name), said the trade is widespread in the northern rural areas of Aleppo and Idlib.

"A dermatologist asked me to sell the organs of pro-government detainees in rural Idlib, since, as he put it, they were going to be executed anyway," said the doctor, who specializes in surgical oncology and traveled to Syria recently to treat patients in the rural areas around Idlib.

The dermatologist explained to him that there were many buyers who were willing to pay, and that the money would be used to buy much-needed medical equipment and to support Syria's armed opposition groups.

Awram refused on ethical grounds. He was also worried that such operations might lead to innocent people being arrested in order to harvest their organs. His refusal led to accusations that he was working for the Syrian government.

The Islamic State, he said, tried to kill him several times when he attempted to manufacture medicines, so he moved to rural Aleppo when Idlib was overrun with the Islamic extremists.

"The area I moved to was also controlled by the Islamic State, and we saw many cases of corpses with missing internal organs, mostly the liver and left kidney. However, I saw one case of a missing bladder," he said.

Murhaf al-Muallem, director of the Consultative Center for Studies and Human Rights, said his organization has documented dozens of cases of Syrian organs being sold inside and outside Syria.

"The center blames Syria's neighboring countries for the situation, since they are not providing Syrian refugees with protection or job opportunities, which has led many of them to sell their own organs in order to provide for their families. Their poverty made them easy victims for the organ trade mafias," he said.