Ted Cruz has caused a big upset by beating Donald Trump in the Republican Iowa caucuses, while Hillary Clinton claimed an extremely narrow victory over rival Bernie Sanders in the Democrats' race.
The caucus is the first vote in the race for the US presidential nominations for both parties, and is seen as an important bellwether for candidates' chances in the elections.
Polls show former Secretary of State Hillary Clinton and US Senator Bernie Sanders are too close to call in Iowa.
The Democrat rivals came in deadlocked both receiving roughly 50% in what Sanders, a self-described democratic socialist, declared as a "virtual tie."
The party said it was awaiting results from a "small number of outstanding precincts" and had reached out to the campaigns for help contacting the chairs from those sites.
ITV News Political Editor Robert Peston said the tie was a "bigger story" than Donald Trump falling into second place in the caucus - noting a "huge surge" by the man he described as "America's Jeremy Corbyn".
Bernie Sanders' brother says he thinks his sibling has what it takes to win the next US Presidency.
There is much still to play for in both parties in the first real test for America's would-be presidents.
Ted Cruz caused a huge upset by beating Donald Trump in Iowa's Republican caucuses.