US President Barack Obama has warned White House contenders to avoid raising tensions, a day after a rally by Donald Trump was called off amid clashes.

Mr Obama said candidates should not resort to “insults” and “certainly not violence against other Americans”.

Mr Trump, who leads the race for the Republican nomination, cancelled his Chicago rally after fighting broke out between his supporters and protesters.

His rivals and others have accused him of using inflammatory rhetoric.

Later on Saturday, Mr Trump suffered heavy defeats in Republican caucuses in Washington DC and Wyoming.

Mr Obama, who will be standing down next January following November’s presidential election, was speaking at a Democratic Party fundraiser in Dallas on Saturday.

He said: “What the folks who are running for office should be focused on is how we can make it even better – not insults and schoolyard taunts and manufacturing facts, not divisiveness along the lines of race and faith.”

The clashes at Mr Trump’s Chicago rally on Friday began more than an hour before the event was due to start, and continued after it was cancelled.

‘Tremendous anger’

On Saturday Mr Trump campaigned in Ohio, one of several key states - also including Florida and Ohio - holding primaries on Tuesday.

In Dayton, Ohio, he was briefly surrounded by Secret Service agents on stage after a man tried to breach the security cordon.

Mr Trump has taken a strong anti-immigrant stance, promising to build a “great wall” at the border with Mexico.

Commenting on relations between Muslims and America earlier this week, he said: “Islam hates us.”

Speaking to Fox News after Friday’s events in Chicago, Mr Trump denied fostering division.

“I represent a large group of people that have a lot of anger,” he said. “There is tremendous anger out there on both sides.”

Mr Trump’s rivals for the Republican nomination, Marco Rubio and Ted Cruz, have both called the incident “sad”.

Texas Senator Cruz accused Mr Trump of creating “an environment that only encourages this sort of nasty discourse”.

Mr Rubio and another Republican challenger, John Kasich, suggested they might not rally behind Mr Trump if he wins the nomination.

Mr Rubio said it was “getting harder every day” to keep his promise to unite behind the eventual Republican nominee.

Mr Kasich said Mr Trump’s rhetoric “makes it very difficult” to support him.

On Saturday, Mr Cruz won a convincing victory in the Wyoming caucus, while Mr Rubio narrowly defeated Mr Kasich in Washington DC. Mr Trump came a distant third in both contests.

Mr Cruz also won on the island territory of Guam.

In the Democratic race. Vermont senator Bernie Sanders is continuing his challenge against frontrunner Hillary Clinton.

Mrs Clinton won the first ever Democrats’ vote in the Northern Mariana Islands. (BBC)

•Photo shows US President Obama.