US President Barack Obama said Wednesday he would take a break from politics upon leaving office, but would not hesitate to speak up if America's "core values" are questioned, citing concerns about discrimination and freedom of the press.

In a veiled warning to his successor Donald Trump, Obama also said systematic discrimination or any effort to "round up" immigrant youths "arbitrarily or because of politics... would merit me speaking out."

"There's a difference between that normal functioning of politics and certain issues or certain moments where I think our core values may be at stake," Obama said.