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.
US Republican presidential candidate Rand Paul has suspended his 2016 campaign.
The announcement comes two days after the Iowa caucuses, where the Kentucky senator finished in fifth place with 4.5 percent.
Paul said in a statement: "It's been an incredible honor to run a principled campaign for the White House. Today, I will end where I began, ready and willing to fight for the cause of Liberty."
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.