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.

US hunts missing Americans after Mexico kidnapping

The unidentified US citizens crossed the border to purchase medication, Mexico's president says.

^{By Madeline Halpert & Will Grant, Mexico correspondent (BBC News)}