Former US President Donald Trump has criticized Vice President Kamala Harris over the Democratic Party's immigration policy during a visit to the southern border with Mexico.

Trump held a news conference in a border county in the western state of Arizona on Thursday.

He said that Harris wants the US "to be open to the world's criminals." He added that "for nearly four years as border czar, Harris has overseen a nation-wrecking border invasion."

The Republican presidential candidate went on to say, "We will seal the border, stop the invasion, and launch the largest deportation effort in American history, we will impose tough new sentences on illegal alien criminals."

Trump is stepping up his campaign against the Democrats by touring key battleground states.