Washington DC - President-elect Donald Trump confirmed Monday that he plans to declare a national emergency on border security and use the US military to carry out a mass deportation of undocumented migrants.

Immigration was a top issue in the election campaign, and Trump has promised to deport millions after record numbers of migrants crossed without documentation during President Joe Biden's administration.

On his social media platform Truth Social, Trump amplified a recent post by a conservative activist that said the president-elect was "prepared to declare a national emergency and will use military assets to reverse the Biden invasion through a mass deportation program."

Alongside the repost, Trump commented, "True!"

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Trump sealed a remarkable comeback to the presidency in his November 5 defeat of Democratic Vice President Kamala Harris.

He has been announcing a cabinet featuring immigration hardliners, naming former Immigration and Customs Enforcement acting chief Tom Homan as his "border czar."

Homan appeared at the Republican National Convention in July, telling supporters, "I got a message to the millions of illegal immigrants that Joe Biden's released in our country: You better start packing now."

Trump's deportation plan is expected directly to impact around 20 million families.

Trump has super-charged concerns with racist claims that an "invasion" is underway by migrants he claims will rape and murder Americans.

During his campaign, Trump repeatedly railed against undocumented immigrants, employing incendiary rhetoric about foreigners who "poison the blood" of the US and misleading his audiences about immigration statistics and policy.

Trump has not elaborated on his immigration crackdown in any detail but during his election campaign repeatedly vowed to invoke the Alien Enemies Act of 1798 to speed up deportations.

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Critics say the law is outdated and point to its most recent use during World War II to hold Japanese-Americans in internment camps without due process.

The number of US border patrol encounters with migrants crossing from Mexico without documentation is now about the same as in 2020, the last year of Trump's first term, after peaking at a record 250,000 for the month of December 2023.