Immigrant deaths at the US border increase by 55%.

Reuters The number of migrants dying from extreme heat at the U.S.-Mexico border has risen 55 percent in the last nine months, following a surge in the number of families and unaccompanied children attempting to enter the U.S. illegally, the U.S. government said Monday.

Heat-related deaths, the leading cause of migrant fatalities on the southwestern border of the United States, have reached 48 this year, compared to 31 during the same period in 2017, according to U.S. border security agency spokesman Salvador Zamora.

The number of deaths is expected to rise with the heat of the summer months, as vulnerable migrants attempt to cross arid environments, paving the way for a rise in border fatalities in 2018.

The border agency recorded a 12 percent year-on-year increase in migrant arrests in the eight months ending May 31, Zamora said.

"We are about to surpass last year's heat-related deaths, and summer is only just beginning," he said in a telephone interview. "The demographics of the illegal immigrants we are apprehending—the family units, the unaccompanied children—they are much more vulnerable."

Humanitarian groups, such as Border Angels, say that the main cause of the increase in deaths is tighter border security and measures such as the "zero tolerance" policy, which has led migrants to choose longer routes through hostile terrain and remote crossing points.

"We've seen people passing through more dangerous areas, so although fewer people are entering, more people are dying," said Enrique Morones, founder of the group that distributes water to immigrants.