Farmington Hills: A 21-year-old man with both US and Syrian citizenship has been missing for three weeks after travelling to Syria from Detroit, a family member said Monday.

Obada Mzaik flew to Damascus from Detroit Metropolitan Airport on January 3 but wasn't seen leaving an immigration checkpoint, said Dr Firas Nashef, his uncle in Farmington Hills.

Mzaik, a native of Columbus, Ohio, was planning to study civil engineering at Al Yarmouk University. He had been temporarily living in the Detroit area with a younger brother while taking fall classes at Oakland Community College, Nashef said.

The brother, Obaie Mzaik, 19, was travelling with Obada but was not detained in Damascus, he said.

"We have no idea what happened," the dentist said. The State Department said it's aware of the matter and is working with authorities in Syria.

On January 11, the US government urged Americans to get out of Syria and to avoid any travel there.

Mzaik has citizenship in the US and Syria. Nashef said the family returned to Syria in 1994 when his nephew was about 3. "Most of his life he's been in Syria," Nashef said.