Alejandro Dominguez was sprayed by at least 30 bullets after attackers ambushed his car in Nuevo Laredo late on Wednesday, witnesses said.

Dozens of people have been killed in the region in a wave of drug-related violence since the authorities launched a crackdown on drug gangs in January.

The US has warned citizens about the risks of travelling in the border area.

'Mother of all battles'

The attackers - who arrived in three vehicles - opened fire as Dominguez was getting in his car on Wednesday evening, a witness was quoted as saying by the Associated Press news agency.

"They cut him off so he couldn't go. They shot him from inside [the vehicles] and then got out to shoot him more," the witness said.

He added that the gunmen then drove away slowly, with their lights turned off.

Dominguez had taken office earlier in the day, saying he was not afraid of any threats.

"I don't owe anybody anything. My duty is to the citizenry," he said.

More than 50 people have been killed since January in Nuevo Laredo - a major transit point for drugs entering the US.