Ciudad Victoria (Mex), Mar 7 (AP) Gunmen kidnapped four US citizens who crossed into Mexico from Texas last week to buy medicine but were caught in a shootout that killed at least one Mexican citizen, US and Mexican officials said on Monday.

The four were in a white minivan with North Carolina licence plates. They came under fire on Friday shortly after entering the city of Matamoros from Brownsville, the southernmost tip of Texas near the Gulf coast, the FBI San Antonio Division office said in a statement on Sunday.

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"All four Americans were placed in a vehicle and taken from the scene by armed men," the office said. The FBI is offering a USD 50,000 reward for the victims' return and the arrest of the kidnappers.

Mexican President Andrés Manuel López Obrador said on Monday the four were going to buy medicine, "there was a confrontation between groups, and they were detained," without offering details.

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A woman driving in Matamoros saw what appeared to be the shooting and abduction. She asked not to be identified for fear of reprisal.

The scene illustrates the terror that has prevailed for years in Matamoros, a city dominated by factions of the Gulf cartel who often fight among themselves. Amid the violence, thousands of Mexicans have disappeared just in Tamaulipas state, where Matamoros is located.

The woman said she saw the white minivan get hit by another vehicle near an intersection, then gunfire rang out.

Another SUV rolled up and several armed men hopped out.

"All of a sudden they (the gunmen) were in front of us," she said. "I entered a state of shock, nobody honked their horn, nobody moved. Everybody must have been thinking the same thing, if we move they will see us, or they might shoot us."

She said the gunmen forced a woman, who was able to walk, into the back of a pickup truck. Another person was carried to the truck by the gunmen but could still move his head.

"The other two they dragged across the pavement, we do not know if they were alive or dead," she said.

Mexican authorities arrived minutes later.

A video posted on social media on Friday shows men with assault rifles and tan body armour loading the four people into the bed of a pickup truck in broad daylight. One was alive and sitting up, but the others appeared to be either dead or wounded.

Shootouts in Matamoros on Friday were so bad that the US Consulate issued an alert about the danger and local authorities warned people to shelter in place. It was not immediately clear how the abductions could have been connected to that violence on Friday.

US Ambassador to Mexico Ken Salazar said in a statement on Monday that the Americans were kidnapped at gunpoint and an "innocent" Mexican citizen died in the attack. He said various US justice agencies were working with their Mexican counterparts to recover the missing. Authorities have provided no other details about who the victims were or where they were from.

President Joe Biden had been informed of the situation, White House Press Secretary Karine Jean-Pierre said on Monday. She declined to answer other questions, citing privacy concerns.

Tamaulipas state police said people had been killed and injured on Friday, but not how many. The state police said on social media that neither law enforcement nor the military were involved in "two armed incidents between unidentified civilians".

Victims of violence in Matamoros and other large border cities of Tamaulipas often go uncounted, because the cartels have a history of taking the bodies of their own with them. Local media often avoid reporting on such incidents out of safety concerns, creating an information vacuum.

Photographs from the scene viewed by The Associated Press show a white minivan with the driver's side window shot out and all of the doors open, sitting on the side of the road after apparently colliding with a red SUV. Multiple people were lying in the street beside it surrounded by rifle-toting gunmen.

Their positions appeared to correspond with the video posted online that was taken from another angle, which showed them being dragged across the street and loaded into the bed of a white pickup. One person who was sitting up in the street walks under their own power to the pickup. At least one other appeared to lift his head from the pavement before being dragged to the truck.

Tamaulipas state's many border crossings with Texas make it lucrative for the cartels that move drugs, migrants and guns between Mexico and the US.

The State Department's travel warning for Tamaulipas warns US citizens not to travel there. However, being a border city, US citizens who live in Brownsville or elsewhere in Texas frequently cross to visit family, attend medical appointments or shop. It would also be a crossing point for people travelling deeper into Mexico.

Previously, as the headquarters of the powerful Gulf cartel, Matamoros was relatively calm. For years, a night out in Matamoros was also part of the "two-nation vacation" for spring breakers flocking to Texas' South Padre Island.

But increased cartel violence over the past 10 to 15 years frightened away much of that business. Sometimes US citizens are swept up in the violence.

Three US siblings disappeared near Matamoros in October 2014 and were later found shot to death and burned. They had disappeared two weeks earlier while visiting their father in Mexico. Their parents said they had been abducted by men dressed in police gear identifying themselves as "Hercules", a tactical security unit in the violent border city. (AP)

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