CNN and CBS rapidly predicted the result in Trump's favor over former UN ambassador Nikki Haley and Florida Governor Ron DeSantis, although his victory margin was not immediately clear.

Bundled up Iowa residents had shuffled into more than 1,600 voting locations which opened at 7:00pm (0100 GMT), launching a blockbuster evening seen as a test of Trump's promise to ride a year of enormous polling leads to a stunning White House return.

The Hawkeye State endured its coldest ever caucuses, with wind chills of -40 degrees (Fahrenheit and Celsius) forcing candidates to cancel events at the last minute and aides to fret over turnout. 

In the end, the networks were able to call the state-wide contest barely 30 minutes after the caucus doors opened.

Trump hopes to lock in victory as the Republican nominee by crushing Haley and DeSantis by a large margin.

Iowa accounts for less than two percent of the delegates awarded nationwide in the process to pick a party flagbearer, so a big night by no means guarantees success in the rest of the nominating season. 

But a strong showing is essential for candidates hoping for a springboard to New Hampshire, Nevada and South Carolina, and onwards to the White House.

Democrat President Joe Biden on Monday acknowledged that Donald Trump was the "clear front runner" to be the Republican candidate in the US election after Trump won the Iowa caucuses.

"Looks like Donald Trump just won Iowa. He's the clear front runner on the other side at this point," Biden said on X, formerly Twitter.

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