Mexico hopes to have up to 7,000 troops in the city in a few days
More than 1,500 Mexican troops have moved into a city on the US border being fought over by rival drug gangs.
Soldiers moved into Ciudad Juarez to try to regain control of a city in which more than 2,000 people have been murdered over the past year.
Officials say they intend to have 7,000 troops and police in position by the end of the week.
Rival gangs are battling for control of the city, which is a key entry point for drug smuggling into the US.
Attorney General Eduardo Medina Mora said the troops were there in support of the local authorities.
"Ciudad Juarez worries us deeply," he said in an interview with Reuters news agency.
"It is the reason why there is a response by the federal government to support the request of local authorities. Public safety is a shared responsibility among the federal, state governments and municipalities."
He added: "In areas where drug traffickers have a lot of influence, sadly there is a risk that they will have an interest in influencing the formation of public power, particularly the local authority. This is something that concerns us."
Police chief quit
Last month, the police chief in Ciudad Juarez, Roberto Orduna, stepped down after drugs gangs threatened to kill at least one police officer every two days until he quit.
Mayor Jose Reyes had earlier insisted the city would not back down to criminal gangs.
But after a police officer and a prison guard were killed, he said Mr Orduna's departure was the only way the authorities could protect policemen.
Speaking after the latest deployment of troops in his city, Mr Reyes said the army and the local authorities were working together to help the troops work with local police and residents.