U.S. President Donald Trump's eldest son appeared to liken migrants to dangerous animals, his latest controversial remarks on desperate people seeking to enter the United States.

"You know why you can enjoy a day at the zoo? Because walls work," Donald Trump Jr. wrote in an Instagram story post.

Trump's father has doubled down on his pledge to build a wall on the border with Mexico, leading to a partial shutdown of the U.S. government.

The president himself has described some migrants in similar terms, saying in May last year that: "You wouldn't believe how bad these people are, these aren't people, these are animals."

The Instagram post by Trump Jr — who has previously drawn fire for comparing Syrian refugees to a bowl of potentially deadly Skittles candy — drew criticism on social media.

"This is racism on steroids," one user wrote on Twitter, while another tweeted: "The only zoo animal is Junior himself."