MCALLEN, Texas (REUTERS) - The FBI is searching for a United States (US) Marine reservist who served in Iraq, his father and uncle after they were kidnapped at a ranch in Mexico last month, authorities said on Tuesday.

The FBI has asked for the public's help to locate US Marine Armando Torres III, who was taken with his father Armando Torres II and his uncle Salvador Torres on May 14 in La Barranca, Mexico, near the southern tip of Texas.

The FBI says it is working with Mexican law enforcement agencies to locate Torres, a US citizen, and the two older men, who are both Mexican citizens.

Investigators in Mexico say a property dispute over the partial sale of the ranch may have triggered the kidnapping.

Armando Torres III drove into Mexico on May 14 to visit his father's ranch, the FBI said in a statement.

Shortly after his arrival, gunmen entered the property and kidnapped the three Torres men, who have not been heard from since.

"We'll take any leads to find this guy safely," said Mr Erik Vasys, an agent from the FBI's San Antonio office.

Officials at the state attorney general's office in Tamaulipas, where the ranch is located, said the kidnappings appeared to be linked to a dispute between Mr Torres' family and a man to whom his grandfather had sold part of his ranch.

After the grandfather died, Mr Armando Torres II and his brother Salvador decided to renege on the sale, angering the man, who vowed to settle the matter, the officials said, speaking on condition of anonymity.

They added that it was unclear where the kidnap victims were or who had taken them. Tamaulipas is one of Mexico's most lawless states, with much criminal activity going unreported or uninvestigated for fear of reprisals.

"It's just shocking, you know, because you don't want to believe it," Mr Torres' sister, Ms Cristina Torres, told ABC News. "My cousin called me. She told me you know that she had seen a white truck outside the house. She saw several people just get out of the car and go inside the house and they grabbed my brother and my uncle and my father and just took them." Ms Cristina Torres did not immediately return requests for comment from Reuters on Tuesday.

Mr Armando Torres III is a member of the US Marines in the Individual Ready Reserve - a category of former active duty or reserve military personnel who do not regularly drill, but may be called on to serve - and served in Iraq.