Four Americans kidnapped at gunpoint in Matamoros, Mexico

White House says it is following the situation closely

Matamoros has been racked by cartel violence in recent years. Here, Mexican law enforcement agents guard the entrance to an area where human remains are often found. AFP

Four Americans are missing in Mexico after unknown assailants opened fire on their vehicle shortly after they entered the border city of Matamoros, the FBI has said.

“All four Americans were placed in a vehicle and taken from the scene by armed men,” the FBI reported.

American authorities are asking for the public’s help in identifying the gunmen, as they try to locate the kidnapped US citizens who were ambushed in broad daylight.

The White House said on Monday that it was following the situation closely.

"Our thoughts are with the families of these individuals and we stand ready to provide all appropriate consular assistance, and US law enforcement is in touch with Mexican law enforcement," White House spokeswoman Karine Jean-Pierre said.

Officials believe the four may have been taken by mistake, according to CNN.

The US ambassador to Mexico, Ken Salazar, said that an "innocent Mexican citizen was tragically killed" in the incident.

"US law enforcement officials from numerous agencies are working with Mexican authorities at all levels of government to secure the safe return of our compatriots," Mr Salazar said.

Matamoros, a sprawling city in the northern state of Tamaulipas, is over the border from Brownsville, Texas, where billionaire Elon Musk has built his SpaceX launch facility.

The city has been racked by cartel violence in recent years, with frequent clashes between the Gulf Cartel — which has historically controlled it — and the Zetas, who broke away in 2010.

In recent years, Mexican authorities have also clashed with the cartels, engaging in shoot-outs in the street.

The US State Department warns US citizens against travelling to Tamaulipas because of the risk of kidnapping.

“Criminal groups target public and private passenger buses, as well as private automobiles travelling through Tamaulipas, often taking passengers and demanding ransom payments,” the State Department's travel advisory says.

Matamoros is also a centre for migrants seeking to enter the US, who are often preyed on by the cartels.

Felicia Rangel-Samponaro, a US citizen who travels to Matamoros daily to work with migrant children, said the incident was a scary reminder of how dangerous the city can be.

“Usually these crimes happen against asylum seekers,” Ms Rangel-Samponaro told The National. “So most Americans don't hear about it.”

The FBI has announced a $50,000 reward for the return of the four Americans.

Violence breaks out in Mexico cartel stronghold — in pictures