Donald Trump won primary elections in Virginia and North Carolina, US networks predicted as results began to come in from the 15 "Super Tuesday" states in the race for the Republican presidential nomination.

The former president, who is bidding for a sensational comeback after being unseated by Democrat Joe Biden in 2020, is expected to claim a clean sweep of states Tuesday on the way to the nomination.

Trump thanked Virginia and North Carolina on his Truth Social site, as counts were underway in states across the country.

His longshot challenger, former UN ambassador Nikki Haley, has failed to provide a significant obstacle in Trump's path to the nomination, but has refused to drop out of the race.

Polling averages from RealClearPolitics show 77-year-old Trump 65 points clear in the primary, and two points ahead of President Joe Biden in a one-on-one match-up in the November election.