A photo of a four-year-old Syrian refugee has gone viral on social media after claims he was wandering the desert in search of his family when he was really just metres behind them.

Several media retweeted the picture posted Sunday by UN refugee official Andrew Harper, saying the boy had crossed the desert on his own after being separated from his family and then been reunited with them.

The picture showed the boy, named Marwan, clutching a plastic bag in his hand as UN workers came to help him.

But Harper, the UN High Commissioner for Refugees representative in Jordan, said Marwan had only been "temporarily separated" from his family.

"Marwan was safely reunited with his mother soon after being carried across the Jordan border," Harper wrote on Twitter.

"It was never stated that he was alone. News agencies stated that," Harper wrote.

Another photo shows Marwan's family being only several steps ahead of him, with Harper stressing "he is separated, he is not alone."

"Marwan was not crossing alone; he was crossing with a wave of refugees. He was just at the back of the group," Harper told AFP, adding he had been separated for "minutes."

"When you have a group of refugees crossing you often have the most vulnerable lagging behind, so you have the people with wheelchairs, the women, the elderly, the sick and often the very young."

"When our team saw the young boy there... many other vulnerable people went there and helped him," he added.

"The mother had other children and other luggage to carry and she had her both hands full so sometimes it's difficult to track all your children at one time. Marwan reunited with his family and we moved on."

Some 600-1,000 others also crossed with Marwan into Jordan, which is home to more than 500,000 Syrian refugees.

Marwan will be settled in the Zaatari refugee camp, home to more than 100,000 Syrians, Harper said.