Mexican President Andres Manuel Lopez Obrador confirmed on Monday reports that four US citizens had been kidnapped in northern Mexico.

The incident occurred in the city of Matamoros in the northern state of Tamaulipas, directly across from Brownsville in the US state of Texas.

The San Antonio division of the US Federal Bureau of Investigation reported the kidnapping on Sunday. According to the FBI, the four Americans were travelling in a white minivan with North Carolina license plates.

What else did US and Mexican officials say?

"All four Americans were placed in a vehicle and taken from the scene by armed men," the FBI said.

The FBI is offering a $50,000 (€47,000) reward for the return of the victims and the arrest of the perpetrators of the kidnapping.

Lopez Obrador said the US citizens had crossed the border to buy medicine.

"According to the information we have," the four Americans "crossed the border to buy medicine and there was a group confrontation and they were detained," Lopez Obrador said.

Lopez Obrador said the FBI was working alongside Mexico's Public Security Ministry in investigations. "The whole government is working on it," he said.

The US's ambassador to Mexico, Ken Salazar, said officials from various US agencies were "working with Mexican authorities at all levels of government in order to ensure the victims' safe return."

Salazar said the Americans were kidnapped at gunpoint and an "innocent" Mexican citizen died in the attack.

The state of Tamaulipas has seen a wave of violence amid disputes between rival factions of the Gulf cartel.

On Friday, the US consulate issued an alert about the danger in Matamoros and local authorities warned people to take shelter.

US citizens living in Texas frequently go to Tamaulipas to visit family or shop.

sdi/ar (AP, EFE)