A search is under way for four US citizens who were kidnapped in north-eastern Mexico last week in a clash which left a Mexican bystander dead.
The Americans 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 says.
Mexican President Andrés Manuel López Obrador says the Americans crossed the border to buy medication.
But his statement was not confirmed by US officials, who have not named the four people missing.
President López Obrador said his “entire government” was working to obtain the release of the four US citizens.
He held talks with US Ambassador Ken Salazar at the National Palace in Mexico City on Monday.
“There was a confrontation between groups and they were kidnapped,” the president said. The incident also resulted in the death of an innocent Mexican citizen, Mr Salazar said.
“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.”
The White House was closely following the “unacceptable” assault and kidnapping, press secretary Karine Jean-Pierre said. “Our thoughts are with the families of these individuals,” she added.
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 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 news agency three men and one woman had been kidnapped.
Tamaulipas is one of six states in Mexico that the US state department advises travellers not to visit because of “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 reward for information leading to the return of the missing US citizens and the arrests of the kidnappers.
The Americans were driving in a van with North Carolina licence plates, according to the FBI.
Matamoros is located directly across the border from Brownsville, Texas.
According to the US 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.