US President Donald Trump once again hinted at the idea of serving a third term, saying he was "not 100 percent sure" he was barred from doing so under the Constitution, which forbids it.

"I've raised a lot of money for the next race that I assume I can't use for myself, but I'm not 100 percent sure, because I don't know... I think I'm not allowed to run again," Trump told an audience of Congressional Republicans in Miami.

To laughter, Trump turned to Republican House Leader Mike Johnson and added: "I'm not sure, am I allowed to run again? Mike? I better not get you involved in that argument."

Trump was inaugurated for his second spell in the White House a week ago, becoming just the second president in US history to serve two non-consecutive terms.

US presidents are limited to two terms in office by the 22nd Amendment to the Constitution, which was ratified in 1951 -- partly as a response to Franklin D. Roosevelt's unprecedented four terms as president from 1933-1945.

A Republican in the US House introduced a super-long-shot resolution last week to change the constitution to allow Trump to get another term.

Trump has alluded to extending his stay on a number of occasions and joked about it as recently Saturday, during a rally in Nevada.

"It will be the greatest honor my life to serve not once, but twice -- or three times or four times," he said with a laugh, before adding to cheers from the audience: "Headlines for the fake news."

In November, in another speech to House Republicans shortly after his election win, Trump said: "I suspect I won't be running again unless you say, 'He's good, we got to figure something else.'"

Trump told an audience of conservative Christians in July: "Christians, get out and vote. Just this time... Four more years, it'll be fixed, it'll be fine, you won't have to vote anymore."