Former US Ambassador to UN Nikki Haley has announced her withdrawal from the US presidential campaign.

It comes after Donald Trump swept almost all of the Republican primaries on Super Tuesday, paving the way for a faceoff with President Joe Biden in November.

Haley said: "I am filled with gratitude for the outpouring of support we received from all across our great country. But the time has now come to suspend my campaign. I said I wanted Americans to have their voices hear. I have done that. I have no regrets. And although I will no longer be a candidate, I will not stop using my voice for the things I believe in."

Haley stopped short of endorsing Trump in her brief speech Wednesday in her home state of South Carolina. She said she has always supported the Republican nominee, but it is up to Trump to earn the votes of the party.

Her exit makes it certain the former President will win the Republican nomination.

Haley was Trump's last remaining rival in the race. She captured Vermont on Tuesday but it was just her second win, along with Washington D.C.

Trump has secured 14 out of 15 states including California and Texas, the two most populous in the country. Speaking to supporters, Trump hailed his victories and attacked Biden.

Trump said: "He's the worst president in the history of our country. There's never been anything like what's happening to our country. We wouldn't have Russia attacking Ukraine. We wouldn't have Israel being attacked."

On the Democratic side, Biden carried all 15 states, losing only in the US Territory of American Samoa.

Reacting to the Republican results, Biden said in a statement, "Are we going to keep moving forward or will we allow Donald Trump to drag us backwards into the chaos, division and darkness that defined his term in office?"

Trump is also facing four criminal trials and multiple civil cases that could hinder his campaign. He may have to use election funds to pay legal expenses.

Recent polls suggest some Trump supporters may change their allegiance if Trump is convicted in the criminal cases.

Meanwhile as the oldest president in US history, Biden's physical and mental fitness have been watched closely ever since he took office. Some voters have voiced concerns over his ability to effectively serve another four-year term.