A photo tweeted on Feb 17, 2014, by UNHCR representative Andrew Harper with the caption, "UN staff found 4 year-old Marwan crossing desert alone after being separated from family fleeing Syria." UNHCR/Twitter

In the middle of the vast, rocky desert, a little boy, separated from his family, looked to be wandering alone, carrying all his worldly possessions in a plastic bag half his height, United Nations officials said.

The stark, heart-breaking photo shows the boy, 4-year-old Marwan, being met by blue-vest United Nations aid workers. It was captured by Andrew Harper, the United Nations Refugee Agency's coordinator in Jordan, who posted it on Twitter before it shot around the Internet on Monday. Though the photo makes it look like he was all alone, he had been separated from a larger group in what Harper called "the chaos of the mass refugee [crossing.]"

"Unfortunately in every mass refugee [crossing] to Jordan, it is the elderly, sick, pregnant & often children that fall behind the main groups," Harper tweeted after the image went viral.

Marwan had just crossed Syria's southern desert border on Sunday into northern Jordan, joining the ranks of the almost 600,000 Syrian refugees registered with the U.N. who have fled to Jordan to escape the almost three-year war in Syria that has now left more than 140,000 dead.

After the picture was taken, Marwan was reunited with his mother, Harper said.

A photo tweeted on Feb 17, 2014 by CNN'™s Hala Gorani with the caption, "UN staff found 4 year-old Marwan crossing desert alone after being separated from family fleeing Syria." UNHCR/Twitter

After Lebanon and Turkey, Jordan has taken in the most Syrian refugees. Most are women and children who cross the border on foot with the few belongings they can carry. The U.N. and Jordanian army then pick them up and take them to a registration center.

Jordan's sprawling Zaatari camp is home to more than 100,000 refugees, the second-biggest camp in the world and is effectively Jordan's fifth-largest city.