By Tim Gaynor

PHOENIX, Ariz., Aug 9 (Reuters) - A U.S. Border Patrol agent shot and killed a Mexican smuggler who attacked him with a rock as he was crossing illegally into the United States, authorities said on Thursday.

A Border Patrol spokesman said the agent shot the unnamed man as he was trying to take him into custody late on Wednesday, one mile (1.6 km) east of the Bridge of the Americas port of entry in El Paso, Texas.

"The individual attempted to assault the agent with a rock, ... he also had a pair of bolt cutters in his other hand," supervisory agent Ramiro Cordero told Reuters by telephone.

"This caused the agent to discharge his firearm, striking the subject on several occasions," he added.

Cordero said two other illegal border crossers then dragged the injured man back across the Rio Grande river to Mexico, where he died from his injuries.

The agent has been assigned to administrative duties pending an investigation by the FBI, the El Paso Police Department and the District Attorney for the state of Texas, Cordero said.

Mexican police said the dead man was a known people and drug smuggler.

The incident is the latest of several shootings of border crossers that have strained relations between the United States and Mexico.

Earlier this week a judge in Arizona ruled that a Border Patrol agent must stand trial for murder for shooting a Mexican man who crossed the U.S. border illegally from Mexico.

Last year, two Border Patrol agents were prosecuted in Texas for shooting an unarmed Mexican drug smuggler in the buttocks.

They were convicted and sentenced to more than 10 years in jail, becoming a cause celebre among some conservatives and anti-illegal immigration hard-liners.

Mexico's government requested U.S. authorities investigate the latest incident and punish anyone found responsible for the overuse of force.

"The Mexican government expresses a staunch protest against the use of lethal weapons in situations that do not represent a proportional risk," the Foreign Ministry said in a statement. (Additional reporting by Noel Randewich in Mexico City)

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