MATAMOROS, Mexico - A harrowing photo of a Salvadoran migrant and his young daughter who drowned in the Rio Grande at the US-Mexico border became the focus on Wednesday of a US political debate over President Donald Trump's asylum policies.

The picture of Oscar Alberto Martinez, 25, and his 24-month-old daughter Angie Valeria put a renewed focus on the plight of refugees and migrants who are mostly from Central America. The pair had travelled from El Salvador and were seeking asylum in the United States.

US presidential candidate Bernie Sanders called the image "horrific" and said the president's migration clampdown made deaths more likely.

"Trump's policy of making it harder and harder to seek asylum - and separating families who do - is cruel, inhumane and leads to tragedies like this," he wrote on Twitter.

In turn, Trump blamed the Democrats, whom he said were blocking his government's attempts at closing "loopholes" in US law that encourage migrants to apply for US asylum.

"If they fixed the laws you wouldn't have that. People are coming up, they're running through the Rio Grande," he said, referring to the river known as the Rio Bravo in Mexico that forms a large part of the border between the two countries.

"They can change it very easily so people don't come up, and people won't get killed," Trump told reporters.

Record numbers of Central American migrants are reaching the United States this year despite a crackdown by Trump. Many flee their homes in Central America to escape poverty, drought and high levels of criminal violence, much of it carried out by street gangs.

US border patrol agents have apprehended 664,000 people along the southern border so far this year, a 144 percent increase from last year, said Brian Hastings, chief of law enforcement operations for the US Border Patrol. "The system is overwhelmed," he said.

Reuters