Authorities in Texas reported seven migrant deaths on Monday, including those of a woman, two babies and a toddler, showing the danger of extreme summer heat as Central American families surge across the United States-Mexico border.

The woman and three children may have been dead for days before they were found by US Border Patrol near the Rio Grande in South Texas on Sunday, according to a local law enforcement official, who asked not to be named.

They are thought to have died from heat exposure and dehydration in an area about 29 kilometres east of McAllen.

To the west, US Border Patrol agents in the Del Rio area recovered the bodies of two men on ranchland near Carrizo Springs after anonymous calls on June 19 and June 20 alerted them to lost migrants, the agency said in a statement.

Another decomposed body was found June 20 on the riverbank of the Rio Grande near Normandy.

“The extreme temperatures during this time of year can be fatal," said Del Rio Sector Chief Patrol Agent Raul Ortiz.

Arrests of undocumented migrants reached their highest monthly level since 2006 in May, with more than 60 per cent of those apprehended either children or families, many seeking asylum.

The Trump administration has set limits on how many people can claim asylum each day at ports of entry. With months-long waits for interviews, migrant families are instead attempting sometimes risky border crossings to make their claims.

Smugglers put migrants' lives at risk by dropping them off in desolate areas or sending them across the Rio Grande in makeshift rafts.

Increased fencing and security in many areas means migrants who have been deported, and are seeking to evade capture, often take dangerous routes to re-enter the US.

A six-year-old girl from India died earlier this month from heat stroke in western Arizona after smugglers left a group of migrants in a remote desert location.

Three children and a Honduran man are believed to have died in April when their raft overturned on the Rio Grande near Del Rio. Water rescues at the popular crossing point have soared.

Border Patrol reported 283 migrant fatalities on the border in 2018. Human rights activists say the number is far higher as the remains of many migrants are never found and the agency's data does not include all deaths registered by local authorities.