The results come on a day where Brussels faced a major attack claimed by the Islamic State group
Republican Donald Trump and Democrat Hillary Clinton won their parties' primaries in Arizona on Tuesday and maintained steep advantages in the presidential nominations race, despite victories by rivals in other states.
Trump extended his lead over nearest rival Ted Cruz in the all-important delegate race, although the arch-conservative senator from Texas made a night of it by resoundingly winning the Utah caucuses.
Trump coralled all 58 delegates at stake in winner-take-all Arizona, where he left Cruz and Ohio Governor John Kasich in his wake, and amid controversy over very long lines at polling stations.
"Much bigger win than anticipated in Arizona. Thank you, I will never forget!" Trump posted on Twitter.
But Cruz bounced back in neighboring Utah, as he appeared on track to winning the state by more than 50 percent, which means he secures all of its 40 delegates.
Sanders sweeps Idaho and Utah
Clinton's Arizona victory was tempered by Sanders's impressive performance in Idaho, where he won the caucuses by a staggering 78 percent to 21 percent, and in Utah, where he won 79 percent, results which allowed him to cut into Clinton's delegate lead, if only slightly.
They were Sanders's first state victories since 8 March in Michigan.
Sanders praised the "tremendous" voter turnout, saying in a statement that "these decisive victories in Idaho and Utah give me confidence that we will continue to win major victories in the coming contests."
But the delegate maths looked bleak for the democratic socialist from Vermont. Clinton was projected to finish the night with more than 1,700 delegates, compared with about 930 for Sanders.
To win the Democratic nomination, 2,383 delegates are needed.
At this point in the Republican race, Trump's main objective is to amass the 1,237 delegates needed to win his party's nomination outright, and thwart a bid by the party establishment to stop him.
Following Tuesday's votes, Trump stood at 741 delegates, compared to 461 for Cruz and 145 for Kasich, according to a CNN tally.
Republicans renew calls for refugee ban
The primaries come on a day where Brussels faced a major attack claimed by the Islamic State (IS) group, which killed at least 34 people and wounded hundreds more.
Republican presidential candidates seized on the attacks to demand that Muslim refugees be kept out of the United States, blaming Europe's open immigration policies for the outrage.
Frontrunner Donald Trump repeated his call for closing US borders "until we figure out what's going on" - a call Democratic frontrunner Hillary Clinton said was unrealistic.
"Belgium is a horror show right now. Terrible things are happening. People are leaving. People are afraid. This all happened because, frankly, there's no assimilation," he said on NBC News.
Texas Senator Ted Cruz also called for suspending the resettlement of refugees from countries where the Islamic State group or al-Qaeda control territory, saying the administration's plans to bring in tens of thousands of Syrians fleeing the civil war there "makes no sense".
"We need a president who unleashes the full force and fury on ISIS and utterly destroys them. That's the only way to keep us safe," he said, referring to IS by an alternate acronym.
Hillary Clinton, meanwhile, told supporters at a rally after her win in Arizona that the US should not marginalise or discriminate on religious grounds.
Long lines were widely reported throughout the day of voting in Arizona, with primary-goers claiming to wait upwards of four hours in some precincts in the capital, Phoenix. Hundreds were still in line when Clinton and Trump were projected as winners.
Meanwhile, results from the night's contests in Idaho and Utah are still yet to be reported.