Just 16 years old, Mohammed Hamad was heading to war.

The lanky Syrian teenager was joining what United Nations officials warn might be the start of a flood of underage fighters enlisting in rebel ranks. About half of the 200 new recruits who board buses each week to Syria from Jordan’s sprawling Zaatari refu­gee camp are under 18, U.N. officials at the camp estimate.

Hamad said it was his duty to “fight in the name of God to take back the country’’ from government forces.

“If my generation doesn’t take up arms, the revolution will be lost,’’ he said, shortly before boarding a bus for the border on a three-day journey to join rebel forces on the outskirts of his home village in southern Syria.

The flow of fresh troops has helped the U.S.-backed Free Syrian Army replenish ranks rapidly diminished by a series of recent losses.

But it also has prompted unease from U.N. officials, who in an internal report this month warned of growing “recruitment by armed groups, including of under-aged refugees” in Zaatari and across the region, indicating that the rebels may no longer be honoring a pledge to bar fighters younger than 17.

“We are concerned by reports that some groups may be attempting to use Zaatari as a recruitment center, and we are doing everything in our power to make sure it stays a refugee camp and not a military camp,” Andrew Harper, the U.N. refugee agency’s representative in Jordan, said in an interview.

After more than two years of conflict that has already claimed more than 100,000 lives, some rebel commanders defend the use of teenage fighters as inevitable.

“Many of these young men’s fathers and older brothers have died before them,” said Abu Diyaa al-Hourani, commander of a Free Syrian Army battalion outside the Syrian border town of Sheikh al-Maskin. He said that Syrians as young as 15 serve in his 800-man unit, whose average age has plunged to 19, down from 25 not long ago.

“It is only natural for the next generation to carry on the fight,” he said.

Conscription in the Syrian army is compulsory for all males once they reach the age of 18, according to the military.

At the camp, rebel officials say that theirs remains an all-volunteer force and that prospective recruits are carefully vetted. But the officials acknowledged that verifying birth dates may be all but impossible in camps where few refugees have access to birth documents.

“At the end of the day, if they can carry a gun and are willing to fight, who are we to say they can’t?” said Ayman al-Hariri, a member of the Syrian National Council, an umbrella opposition group, who coordinates repatriation from Zaatari, home to more than 100,000 Syrians.

The families of the young fighters receive monthly benefits from the Free Syrian Army, including salaries and even priority in the distribution of food aid and cash assistance within the camp, refugee officials said. In interviews, several parents — some of whom arrived in Zaatari with little more than the clothes on their backs — said those incentives had influenced their family’s decision.

Um Majed al-Homsawi, 45, said she allowed her 17-year-old son, Ahmed, to return to Syria to fight only after recruiters agreed to provide her family with a fully furnished trailer complete with air conditioning, an upgrade from their standard-issue U.N. canvas tent.

“My daughter has asthma and my husband has had three heart operations — we couldn’t survive in a tent in the desert,” Homsawi said as she placed a kettle atop a propane stove in her recently delivered trailer. “My son decided to join the Free Syrian Army not only to save his country, but to save his family.”

Rebel officials say Zaatari’s proximity to major fighting in southern Syria has transformed the camp into the major supplier of fighters for rebel battalions suffering an average loss of 50 fighters per week.

In the camp, rebel officials said, the Foreign Syrian Army carries out background checks, physical examinations, agility tests and “mental and emotional” evaluations of potential recruits.

On a recent day, Syrian National Coalition and Free Syrian Army representatives in a pair of unmarked prefabricated trailers received a long line of teenage camp residents seeking to enlist with rebel forces.

A group of young men whom the recruiters had rejected for being too young, too weak or otherwise unfit for battle gathered outside. But the young men said they remained determined to join the fight.

“I am going to wait here every day until the Free Syrian Army accepts me, or until Sept. 10,” said Ahmed Saeed, a resident of the city of Daraa whose 17-year-old brother left to fight alongside the Free Syrian Army last week.

“Then I will turn 15.”