(Reuters) - The number of migrants dying from extreme heat on the U.S.-Mexico border rose 55 percent in the past nine months after an increase in unaccompanied children and families trying to enter the United States illegally, the U.S. government said on Monday.

FILE PHOTO: A memorial is seen in the desert near Falfurrias, Texas April 2, 2013. REUTERS/Eric Thayer/File Photo

Heat-related deaths, the main cause of migrant fatalities on the U.S. southwest border, rose to 48, up from 31 over the same period in 2017, said U.S. Border Patrol spokesman Salvador Zamora.

The death toll is expected to rise in the triple-digit heat of summer months as vulnerable, unacclimatized immigrants attempt to cross harsh environments, putting border fatalities on track for a year-on-year increase in 2018, Zamora said.

The Border Patrol recorded a 12 percent year-on-year rise in immigrant arrests in the eight months to May 31, Zamora said.

“We are geared up to surpass last year’s heat-related deaths and the summer is just beginning,” he said in a telephone interview. “The demographics of the illegal aliens we are apprehending, the family units, the unaccompanied children, they’re a lot more vulnerable.”

Humanitarian groups such as San Diego-based Border Angels say the main cause of rising deaths is tighter border security and law enforcement, such as the recent imposition of a “zero tolerance” policy for illegal border crossers. That has prompted migrants to make long treks through hostile terrain via remote crossing points.

“We’ve seen people crossing in more dangerous areas, so even though there’s less people crossing, there are more people dying,” said Enrique Morones, founder of the group whose volunteers leave water for migrants.

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Until four years ago, the vast majority of migrants arrested at the border were Mexicans. With improved economic conditions in Mexico, their number has fallen, as have overall arrests on the border, which dropped to 303,916 in 2017, down 26 percent from 2016, according to Border Patrol data.

Immigrants from Guatemala, Honduras and El Salvador now top the list of people arrested at the southern border as gun and gang-related violence drives an exodus from those countries, according to U.S. government data.

As the number of arrests on the border has fallen, the United Nations International Organization for Migration has recorded a rise in migrant deaths for the past four years, with the total reaching 415 in 2017.

Immigrants, including children, from sub-tropical and mountainous areas are arriving at the border poorly nourished, unacclimatized to arid conditions and susceptible to heat exhaustion, Zamora said.

Families apprehended at the border increased five-fold since 2013, while the number of unaccompanied children detained in 2017 was almost double that in 2010, according to Border Patrol data.

James Cordero of Border Angels has seen a rise in the past few months in migrant families trekking through the California mountains he covers, based on articles like diapers, shoes and “Little Mermaid” backpacks left behind.

“You really know someone is leaving a bad situation if they’re willing to risk their lives and their children’s’ lives for a chance to live,” said Cordero, 36, a water drop leader.

Morones suspects Border Patrol agents of destroying the water supplies left by his group. One such act was caught on video in Arizona by another humanitarian group called No More Deaths.

Border Patrol spokesman Zamora said actions shown in the No More Deaths video were “unacceptable” and in no way representative of the agency’s values. The agency conducts its own humanitarian rescues, which rose slightly to 748 people in the eight months through May, he said.