The US citizens were driving through Matamoros in Tamaulipas state in a white minivan when a group of unidentified gunmen shot at them.

They were then placed in a vehicle and taken away, the US government said.

The US has not confirmed Mexican President Andrés Manuel López Obrador's statement saying that the Americans had crossed the border to buy medication.

The missing Americans, who were kidnapped on 3 March, have not been identified.

"There was a confrontation between groups and they were kidnapped," Mr López Obrador said Monday.

The incident also resulted in the death of an innocent Mexican citizen, US ambassador to Mexico Ken Salazar said in a statement.

"We have no higher priority than the safety of our citizens," the ambassador said.

"Officials from various US law enforcement agencies are working with Mexican authorities at all levels of government to achieve the safe return of our compatriots", he continued.

"The FBI, federal partners, and Mexican law enforcement agencies are investigating," the FBI said.

On Monday, press secretary Karine Jean-Pierre said the White House was closely following the "unacceptable" assault and kidnapping.

"Our thoughts are with the families of these individuals," she said.

The Mexican government is likely to prioritise finding the US citizens to avoid a major cross-border issue between the two countries.

Mr López Obrador said during a news conference on Monday that he had been in contact with the governor of Tamaulipas over the weekend about the kidnappings.

"I think it will get resolved," Mr López Obrador said. "That's what I hope."

A Mexican official told Reuters three men and one woman were kidnapped.

Tamaulipas is one of six states in Mexico that the US State Department advises travellers not to visit due to "crime and kidnapping".

It is considered one of the more dangerous parts of Mexico, where drug cartels control much of the territory and often hold more power than local law enforcement.

The FBI is seeking help from the public and offering a $50,000 (£41,620) reward for information leading to the return of the victims and the arrests of those involved.

The Americans were driving in a van with North Carolina licence plates, according to the Federal Bureau of Investigation, which did not identify the US citizens.

Matamoros is located directly across the border from Brownsville, Texas.

According to the State Department, organised crime "including gun battles, murder, armed robbery, carjacking, kidnapping, forced disappearances, extortion, and sexual assault - is common along the northern border," including in the state of Tamaulipas.

Criminal groups have targeted buses as well as cars driving through the state, often taking passengers and asking for ransom payments in an area where local law enforcement has limited ability to respond to crime, the State Department said.

The US government estimates that hundreds of thousands of Americans cross the border into Mexico each year to receive healthcare services, including prescription drugs. The majority of Americans cite cheaper costs as the most common reason to get treatment abroad.