MEXICO CITY, April 26 (Reuters) - Tightening security on the U.S.-Mexican border has sharply cut arrests of illegal immigrants but scores are still dying as they attempt the perilous crossing through deserts and raging rivers.

David Aguilar, head of the U.S. Border Patrol, said on Thursday arrests on the border had fallen by just under a third compared with the same period last year as the agency increased staffing and improved technology on the frontier.

But he said the number of immigrants killed trying to make the trip had barely fallen, only declining 8 percent so far this year to 153 from 166 during the same period last year.

Aguilar, who spoke to reporters at the U.S. Embassy in Mexico City from Washington through a video conference, gave no reason why the number of deaths had remained so high despite a crackdown that saw 1.1 million immigrants arrested last year.

Warning potential immigrants against making the trip as the deadly summer months approach, he said the majority died trying to cross arid expanses like Arizona's hostile Sonora Desert.

When temperatures in the area hit highs of over 104 degrees Fahrenheit (40 degrees Celsius) in coming months, migrants will have about a 50 percent chance of making the trip alive, he said.

The U.S. government is boosting its border-monitoring technology, including trucks with infrared cameras and underground sensors that detect footsteps as well as surveillance drones flying overhead.

Aguilar said he expected the security crackdown to reduce the number of people making desert crossings during the deadliest period between April and September.

But he said there had also been an increase in the number of people drowning in the Rio Grande which separates the two countries and also warned that robberies by people-smugglers or 'coyotes' were increasingly common.

"People who cross into the United States, who put their lives in the hands of smugglers, face great danger," he said. "When you put yourself in the hands of smugglers you entrust yourself to criminals in a very dangerous area of the border."

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