Donald Trump is heading toward a surprise victory in Tuesday's US presidential election, winning one battleground state after another in what would be one of the biggest upsets in American election history.

Trump has so far picked up 232 electoral votes against Hillary Clinton's 209.

The billionaire real-estate tycoon won such key states as Florida, North Carolina and Ohio, and was leading in Wisconsin, Michigan, New Hampshire, Iowa, Utah and Arizona, whose combined electoral votes total 53. That means if this trend holds, Trump will end up winning more than the 270 votes needed to take the White House.

Clinton was leading only in Pennsylvania, Maine, Minnesota and Nevada, whose combined vote total is 40.

The results belie the late-minute pre-election polls that had widely suggested the 69-year-old former first lady, senator and secretary of state is likely to become the first female president in America's history, though her lead over Trump has significantly narrowed after the FBI's recent reinvestigation of her email scandal.

The unexpectedly close race between Hillary Clinton and Donald Trump has also been watched closely by South Korea and other US allies as the results can have profound impacts on their security and economic relations with the most powerful nation in the world.

Clinton and her husband, Bill Clinton, cast their ballots in Chappaqua, N.Y., Tuesday morning.

"It is the most humbling feeling. I know how much responsibility goes with this. So many people are counting on the outcome of this election, what it means for our country. And I'll do the very best I can if I'm fortunate enough to win today," she told reporters after voting.

Trump voted in Manhattan later in the day.

"It's looking very good. Right now it's looking very good. It will be an interesting day. Thank you," Trump said after casting his ballot.

Earlier in the day, he told Fox News, "I see so many hopes and so many dreams out there that didn't happen, that could have happened, with leadership, with proper leadership. And people are hurt so badly."

More than 200 million people have registered to vote, a record that reflects strong interest in a race marked largely by the rise of the 70-year-old real-estate developer and former reality TV show host as well as questions about his qualifications to be commander-in-chief.

Trump has repeatedly come under fire for a series of unrefined remarks and far-fetched proposals, such as his accusations of Mexican immigrants as "rapists" and his idea of building a massive wall to keep them out. Still, he has enjoyed solid ratings in what is dubbed a "Trump phenomenon" that underlines voter anger against the establishment.

The new president, set to be sworn in on Jan. 20, will take over at a time when the US is grappling with the militant group Islamic State, an increasingly provocative North Korea and a host of problems to solve at home, including immigration reforms, job creation and economic inequalities.

The results could also have deep repercussions not only in the US, but also elsewhere in the world as Clinton and Trump differ sharply in the ways they look at security relations with foreign countries, especially allies, and trade deals around the world.

Clinton praises alliances as a "source of strength" that makes the US safer, while Trump views the relations as well as US security commitments to them as a cumbersome burden sucking up taxpayer dollars, something that the country should abandon unless they make economic sense.

Trump even suggested arming South Korea and Japan with nuclear weapons so as to reduce US security burdens.

South Korea has watched the election more closely than ever due to its potential impact on the alliance with the US, a big departure from past elections, when no candidates questioned the value of the alliance with Seoul, and a focus in Korea policy was mostly on how to deal with North Korea.

Trump has also argued that South Korea should pay 100 percent of the cost for 28,000 American troops stationed in the Asian ally to deter North Korean aggression. Many agree that the troop presence is also in line with US interests in a region marked by China's rise.

Seoul currently pays about half the costs, about US$900 million a year.

A Trump victory could also upset the landmark free trade agreement between the two countries as he has denounced the 2012 pact as a "job-killing" deal and a "disaster."

Clinton has not disputed existing free trade deals, but expressed negative views of the Trans Pacific Partnership (TPP), a 12-nation free trade pact awaiting legislative approval, even though TPP is one of President Barack Obama's foreign policy legacies. (Yonhap)