Republicans use Brussels to attack refugees

Republican presidential candidates seized on the terrorist bombings in Brussels Tuesday to demand that Muslim refugees be kept out of the United States, blaming Europe's immigration policies for the carnage.

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 (IS) or al-Qaeda control territory, saying the Obama administration's plans to bring in tens of thousands of Syrians fleeing the civil war there "makes no sense."

However, Cruz went further by calling for police to "patrol" and "secure" Muslim neighbourhoods in the US.

"We need to empower law enforcement to patrol and secure Muslim neighbourhoods before they become radicalised," Cruz said in a statement on Tuesday.

"For years, the West has tried to deny this enemy exists out of a combination of political correctness and fear. We can no longer afford either. Our European allies are now seeing what comes of a toxic mix of migrants who have been infiltrated by terrorists and isolated, radical Muslim neighbourhoods."

The apparently coordinated bomb blasts in Brussels - for which IS claimed responsibility - ripped through the city's international airport and a metro train station, killing about 35 people.

In the United States, the scenes from Brussels added fuel to an already inflamed Republican debate over immigration, Muslims and the conduct of the US-led war against IS in Iraq and Syria.

The attacks came four days after Belgian authorities arrested Salah Abdeslam, the prime suspect in the November 2015 Paris attacks claimed by the IS group.

In the United States, the scenes from Brussels added fuel to an already inflamed Republican debate over immigration, Muslims and the conduct of the US-led war against IS in Iraq and Syria.

"Belgium is no longer Belgium. Belgium is not the Belgium you and I knew from 20 years ago, which was one of the most beautiful and safest cities in the world," Trump told NBC.

Asked what he would say to the American people in the immediate aftermath of a terror attack, he added: "We are going to be very vigilant and tough. We're not going to allow it to happen to our country".

Clinton, who could possibly face Trump in November's general election, countered that it was "unrealistic to say we're going to completely shut down our borders to everyone."

Ohio Governor John Kasich, who has been dubbed as the more moderate of the three remaining candidates in the Republican race, urged Obama to move quickly to examine US vulnerabilities and "dig in and begin to rebuild the intelligence we need worldwide."

"I think Europe popped up its doors without having a proper vetting process," he said, referring to the waves of immigrants from the Syrian civil war that have pushed into Europe.

He faulted Obama for not acting forcefully enough to bring down Syria's President Bashar al-Assad and for failing to establish no-fly zones.

Agencies contributed to this report