Many of the undocumented young people known as Dreamers "are far from angels" and some are "very tough, hardened criminals," President Donald Trump said on Twitter on Tuesday as the Supreme Court gets ready to review the Deferred Action for Childhood Arrivals (DACA) program that allows them to remain in the United States.

The president posted his message on his Twitter account a few hours before the high court begins hearing arguments for and against DACA, which had been established by his predecessor, Barack Obama, to protect young undocumented migrants brought to the US by their parents from deportation.

"Many of the people in DACA, no longer very young, are far from 'angels.' Some are very tough, hardened criminals," tweeted Trump, who - nevertheless - promised to strike a deal with Democrats in Congress to allow Dreamers to remain in the US if the Supreme Court decides to suspend DACA.

The Supreme Court will decide in the coming months if it will uphold Trump's decision to end the program, which was launched in June 2012 by Obama, or extend it to benefit thousands of immigrants who were brought here illegally by their parents when they were very young and do not have visas.

Figures compiled by the US Citizenship and Immigration Service show that through March 2017 some 787,580 young undocumented migrants had been accepted into the program. That figure between January 2018 and April 30, 2019, stood at 425,760.

"President (Barack) Obama said he had no legal right to sign order, but would anyway," Trump said in his tweet.

"If Supreme Court remedies with overturn, a deal will be made with Dems for (those people protected under DACA) to stay!" he added.

Since he announced his presidential candidacy in 2015, Trump has often referred to immigrants as criminals, rapists, drug traffickers and gangmembers.

One of his main campaign promises during the runup to the 2016 election was to build a wall along the US southern border with Mexico to prevent the entrance of undocumented migrants, and to deport huge numbers of undocumented immigrants, calculated to number some 12 million in the US.

Trump used the same phrase - about not being angels - to refer to the Kurds in Syria who, after fighting alongside the US against the Islamic State, were exposed to a Turkish invasion when the president precipitously decided to withdraw US troops from the region.

To be included in DACA, immigrants must fulfill certain requirements, including having been brought to the US before they were 16 years old, having lived in the US since at least 2007 and having obtained a high school diploma or having served in the US armed forces.