Democratic US presidential candidate Hillary Clinton arrives for a campaign rally at Rainier Beach High School in Seattle, Washington March 22, 2016.[Photo/Agencies]

WASHINGTON  - US Republican front-runner Donald Trump and Democratic favorite Hillary Clinton swept to victory in Arizona on Tuesday in contests that were overshadowed by deadly attacks in Brussels that raised security concerns in the United States.

Trump, who has riled establishment Republicans, easily defeated his two rivals for the Republican presidential nomination, US Senator Ted Cruz of Texas and Ohio Governor John Kasich, US television networks projected.

On the Democratic side, Clinton stretched her advantage in the Democratic contest by winning Arizona, routing US Senator Bernie Sanders of Vermont.

The contests in Arizona and Utah took place on the same day that attacks in Brussels killed at least 30 people, adding to Americans' concerns about the threat from Islamist militants.

"This is a time for America to lead, not cower," Clinton told supporters in Seattle in a victory speech she used to attack Trump and Cruz for views she said were out of step with most Americans.

Sanders was unbowed in defeat, saying national polls showed him gaining on former secretary of state Clinton.

"When we began this campaign about 10 months ago we were three percent in the polls, about 70 points behind secretary Clinton. As of today the last poll that I saw we are five points behind and we are gaining," he said.

Long lines of voters were reported in both states.

Trump, the New York billionaire and former reality TV star, has ridden an anti-Washington message to become the favorite for the nomination. This has left a flagging anti-Trump effort with faint hopes of stopping him at the Republican national convention in July.

"Much bigger win than anticipated in Arizona. Thank you, I will never forget!" Trump said on Twitter. "Hopefully the Republican Party can come together and have a big WIN in November, paving the way for many great Supreme Court Justices!"