Two of the four US citizens kidnapped after crossing the border into crime-plagued northeastern Mexico have been found dead, Mexican authorities said Tuesday.

The attorney general's office confirmed that of the four abductees "two of them are dead, one person is injured and the other is alive," Tamaulipas state governor Americo Villarreal told a news conference by telephone.

The US citizens crossed into Matamoros, in Tamaulipas state, on Friday in a white minivan with North Carolina license plates, the US Federal Bureau of Investigation (FBI) said in an earlier statement.

It offered a $50,000 reward for help leading to the return of the unidentified victims and the arrest of the perpetrators.

"Shortly after crossing into Mexico, unidentified gunmen fired upon the passengers in the (minivan). All four Americans were placed in a vehicle and taken from the scene by armed men," the FBI said.

Mexican President Andres Manuel Lopez Obrador said on Monday that the victims were believed to have entered the country to buy medicines and got caught up in a confrontation between criminal groups.

"We're very sorry that this is happening in our country," Lopez Obrador told reporters after the confirmation of the deaths.

"We send our condolences to the families of the victims, to friends, to the people of the United States, to the US government," he added.

A Mexican national was killed in the incident.

"Our thoughts are with the families of these individuals," spokeswoman Karine Jean-Pierre said.

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