GENEVA -- The UN's migration agency said Friday that it has tallied an increase this year in deaths of people trying to enter the United States from Mexico even as illegal crossings appear to have dropped sharply.

The International Organization of Migration counted 232 migrant deaths through the end of July, up from 204 a year earlier.

U.S. authorities have denied entry 140,000 times during the first half of the year, barely half of last year's count, giving a rough sense of how sharply illegal crossings have dropped this year.

Fifty bodies were found in July alone, including 10 discovered in a truck in San Antonio, Texas, the migration agency said.

The Geneva-based agency said the higher toll could not be fully explained. It made no reference to President Donald Trump's calls for tighter border controls, but cited factors like hot weather and swelling Rio Grande waters.

The Border Patrol has counted 156 deaths on the Mexican border during the first seven months of the year, down 19 per cent from 193 during the same period of 2016.

The U.N. agency's tally is based on figures from U.S. county medical examiners and sheriff's offices and media reports from the Mexican side of the border.

Thousands have died crossing the border since the mid-1990s, when heightened enforcement in San Diego and El Paso, Texas, pushed traffic into Arizona's remote, scorching deserts.

In recent years, South Texas has become the busiest corridor for illegal crossings and also the most deadly.

For the first seven months of the year, the Border Patrol reported 80 deaths in its Rio Grande Valley sector and 64 in its Laredo sector -- both in South Texas -- and 49 in its Tucson, Arizona, sector.