Cruz says police must 'secure' US Muslim neighbourhoods:

 Brussels attacks

Mr Cruz, who is currently second behind Mr Trump in the race for the Republican presidential nomination, posted his response to the attacks on Facebook.

After repeating his standard campaign-trail assertion that Barack Obama has failed to confront - or even properly identify - "radical Islamic terrorists", he called for the US to stop admitting refugees from areas with a so-called Islamic State or al-Qaeda presence.

He then turned his attention to the home front.

"We need to empower law enforcement to patrol and secure Muslim neighbourhoods before they become radicalised," he said.

That particular line has prompted a storm of criticism among US commentators.

"Cruz, running for POTUS as a religious freedom purist, advocates we single out neighbourhoods for extra policing based on inhabitants' faith," tweeted Betsy Woodruff of the Daily Beast.

Wired editor Emily Dreyfuss tweeted: "I live in a Muslim neighbourhood. My neighbours spent the weekend cleaning up the playground so our children can enjoy spring. Screw Ted Cruz."

Mr Cruz's post echoes similar remarks by Mr Trump after the Paris attacks in November that the government would have "absolutely no choice" but to shut down some US mosques.

"Some really bad things are happening, and they're happening fast," he said at the time.

Shortly after the news of the Belgium attacks broke, Mr Trump tweeted his first response.

"Do you all remember how beautiful and safe a place Brussels was," he asked. "Not anymore, it is from a different world! US must be vigilant and smart!"

Mr Trump continued on this theme during an interview with Fox News, saying that he would "close up our borders to people until we figure out what is going on". The interviewers did not press the New Yorker on whether he was announcing a new policy of closing US borders entirely or just reiterating his earlier call of a temporary ban on Muslims entering the country.

"We don't learn," he said. "Brussels is an amazing example. Brussels was an absolutely crime-free city. One of the most beautiful cities in the world. And now you look at it, it's a disaster."

"You see what's happening in London and other cities," he continued. "It's not pretty to watch. Many cities will be this way with what's taking place."

On NBC he said Belgium was a "total mess". On CBS he called it a "horrible city".

"They have areas in Brussels where the police can't even go," he said. "The police are afraid to go there. The police don't even go there. It's a mess. And if you look at Paris, believe me it's the same thing."

He backed off on his possible call to close the US to all foreign entries, however, saying that the US has to be "strong and vigilant" at the borders.

Following the Paris attacks in November there was ample speculation in the media that Mr Trump would suffer, as Republican voters took a more serious, measured view of the candidates' foreign policy credentials.

That was back when people were still of the belief that Mr Trump would somehow, some way be toppled from his perch at the top of the Republican pack.

Although fellow front-runner and political outsider Ben Carson's campaign fortunes took a nosedive, Mr Trump solidified his lead in the polls. And now Mr Trump appears hopeful that he'll receive a similar bump after the Brussels bombings.

"I have proven to be far more correct about terrorism than anybody - and it's not even close," he tweeted, before addressing Republicans in the two states holding their primaries today. "Hopefully Arizona and Utah will be voting for me today!"

The Washington Post's Greg Sargent, while not discounting the possibility that Mr Trump will get a boost in the Republican primaries, says the candidate's bellicose response could prove trouble in the general election.

"Trump has so far functioned in a kind of protected environment, where the limitations on what the GOP candidates can say to GOP primary voters have precluded a very aggressive unmasking of Trump's ignorance and demagoguery," he writes. "In the general election, Trump is going to run into a buzz saw of reality, where no such protections exist."

The general election, however, is still a long way off. And this campaign has hardly gone according to the standard political script so far.