A Chinese national has been missing for five weeks after being abducted in California, the US by three men demanding a 2-million-US-dollar ransom, the FBI revealed on Monday.

Ruochen "Tony" Liao, 28, who worked at a luxury car dealership in Costa Mesa, was kidnapped on July 16 in the city of San Gabriel by three men riding in two black vehicles, said Laura Eimiller, spokeswoman for the FBI office in Los Angeles.

She said Liao's abduction may be linked to a business deal gone sour, as he often worked with "people not often reputable."

"A working theory is that there was some sort of business dispute," Eimiller said.

She said the kidnappers, all believed to be Chinese, initially reached out to Liao's family in China, demanding 2 million US dollars for his release, but then failed to follow through with their demand.

"We don't have evidence indicating he has left the country but we don't know where he is," Eimiller said.

She said only one of the abductors, identified as a 35 to 40-year-old by the name of David, was seen by a witness who provided a description.

Liao moved to the Los Angeles area after attending school in the state of Nebraska.

His family is offering a 150,000-US-dollar reward for information leading to his safe return while the FBI is offering 25,000 US dollars for information leading to his recovery.

"He's a deeply loved person by his family," Matthew Lombard, an attorney representing Liao's family, told reporters. "He's their only child and they're very, very concerned for him."