The FBI is investigating a kidnapping of four US citizens who were last seen in Mexico, the US Embassy in Mexico City said Sunday.

The Americans crossed into Matamoros, Tamaulipas, in a white minivan with North Carolina plates on March 3 and came under fire soon after, according to the embassy.

Gunmen opened fire on the van and then pulled all four Americans out and ordered them into another vehicle at gunpoint, officials said.

The FBI is offering a $50,000 reward for the return of the victims and arrests of their kidnappers.

The embassy did not provide further information on the abduction and the victims.

The FBI urged anyone with information to call its San Antonio division at 210-225-6741.

Matamoros is located along the Rio Grande across from Brownsville, Texas.