Donald Trump's threat to begin deporting "millions of illegal aliens" next week triggered heated rhetoric Tuesday on both sides of the country's political divide, with Democrats warning the US president was fear-mongering ahead of his ­re-election campaign launch.

Liberal lawmaker Alexandria Ocasio-Cortez went so far as to accuse the Trump administration of operating "concentration camps" at the border, where a surge of migrants has all but overwhelmed US officials' capacity to detain them.

Trump vowed to have US Immigration and Customs ­Enforcement arrest and remove undocumented migrants "as fast as they come in" - a move that could disrupt thousands of families. The comment thrusts the controversial issue to the fore as the Republican leader and his Democratic rivals ramp up their 2020 presidential campaigns.

The US is facing a surge in migrant arrivals from several Central American countries plagued by gang violence and poverty. The numbers have smothered US authorities' abilities to temporarily shelter and process them.

Trump has called it "an invasion," and has made the fight against illegal migration a central plank of his administration.