Taiz, Yemen - Hamas Yaseen, a 14-year-old from Taiz's al-Masbah neighbourhood, usually returned from his job as a motorcycle driver by 8pm.

But on the night of October 15, he never came back.

His father, Abdullah, had worked as a painter until the civil war came to Taiz, leaving him unemployed. So he bought a motorcycle for Hamas and asked him to help bring in money for the family.

"When my son did not return to the house, I tried to call him, but his mobile was off," said Abdullah. "Then I feared that he was killed by clashes, and I visited most of the hospitals in the city, but I did not find him. Then, I asked all of his friends about him, and they said they did not see him that bad day."

The next day, at 2pm, Abdullah received a message from his son saying: "Dad, don't worry, I am in a safe place, and I will return to the house soon."

Hamas is far from the only child in Taiz province to have joined the fighting. Last April, UNICEF estimated that children comprise up to one third of all fighters in Yemen's civil war, which has so far caused the deaths of about 6,000 people.

At least 505 children have been killed, 702 injured and more than 1.7 million put at risk of malnutrition, according to UNICEF.

Contradictory rumours circulate as to where Hamas is now. Some say he is with the resistance, fighting against the Houthi rebel group that controls large swaths of Yemen.

He allegedly brings qat, a mild narcotic popular in Yemen, from the market to the fighters. But other rumours say that Hamas sold his motorcycle and became a fighter with the Houthis.

"I believe that he joined the fighting, but I cannot say with which one, as some of his friends joined the war with the Houthis and others with the resistance," Abdullah explained.

Abdullah said he regretted not being as involved as he could have been in his son's life. "Hamas' mother still cries until today. She accuses me of being the main reason of losing Hamas, as I bought a motorcycle for him, but I hope that I can see him again in good health."

READ MORE: The child soldiers of Yemen

... some of his friends told me that he was talking with them about the bravery of the resistance fighters in Taiz. Then I understood that he joined the resistance Rafat al-Homaid, Taiz resident

Fifteen-year-old Rafat al-Homaid, a friend of Hamas, disappeared on September 22. "When my brother Rafat did not return home, some of his friends told me that he was talking with them about the bravery of the resistance fighters [forces loyal to President Abd-Rabbu Mansour Hadi] in Taiz. Then I understood that he joined the resistance," said Rafat's brother, Sa'ad.

Sa'ad said he tried to look for his brother in several places before visiting the headquarters of the popular resistance, located at the Zaid al-Moshki school.

"On September 27, I found my brother in a military camp in al-Rodha area, and I took him with me by force after I told the resistance that my mother cannot live without Rafat," Sa'ad explained.

He said that he persuaded his brother about the danger of participating in the war.The fighters allowed Sa'ad to take his brother after getting permission from the leadership.

Jamal al-Shami, the head of the Sanaa-based Democracy School, a non-profit organisation to raise awareness in human rights and democracy among children, told Al Jazeera that the number of the child fighters is increasing.

WATCH: Civilians pay heavy price for war in Yemen

Armed groups find it easy to persuade children to join them, he said, in part due to the dire economic conditions in Yemen. "There are many families in several provinces that deliberately send their children to fight for the sake of money, after these families lost their source of income at the beginning of the war," Shami said.

One of those children is Molham Khalil al-Ameer, aged 15. He joined the Houthis after his father Khalil, who had been a construction worker in Taiz, was unable to find work due to the war.

"I am not a supporter of the Houthis, but I sent my eldest son to fight with them. They pay him 2,000 rials [$9.30] daily, and this is enough for us," Khalil told Al Jazeera.

Khalil added that he himself was thinking of joining the Houthis, but that those over the age of 20 are sent to the frontlines, while children like Molham are based away from the fighting.

Both the Houthis fighters and pro-Hadi forces deploy children at checkpoints and task them with guarding public institutions, but also at times, children get to be dispatched to the frontlines in case of a shortage of fighters.

While the Houthis pay 2000 rials a day, the pro-Hadi forces pay 1000 rials for child soldiers, and 2000 rials for the fighters on the frontline.

In addition to economic reasons, a desire for adventure is causing some children to enter the fray, said Shami. "There are hundreds of children who joined the war secretly without informing their relatives," he said. "And these kind of children usually are brave, and like the adventures."