Democratic presidential candidates have blamed Donald Trump's hard-line immigration policies for the deaths of migrants on the US border after a graphic picture of a father and his daughter lying dead in the Rio Grande shocked the world.

The photograph showed the drowned bodies of Salvadoran refugee Oscar Alberto Martinez Ramirez, 25, and his 23-month-old daughter, Valeria, locked in a final embrace as they attempted to cross the river from the Mexican city of Matamoros to Brownville, Texas.

The image has come to symbolise the large-scale humanitarian crisis on America's southern border and prompted an outpouring of sympathy from across the globe.

Leading Democrats, ahead of the first primary debate of the 2020 race on Wednesday night, were quick to lay the blame for the tragedy directly at the US president's door.

"Trump is responsible for these deaths," presidential candidate Beto O'Rourke bluntly stated in a tweet.