Republican US presidential candidate Donald Trump at a news conference at the construction site of the Trump International Hotel at the Old Post Office Building in Washington, D.C., on Monday. Reuters/Jim Bourg Republican presidential frontrunner Donald Trump on Tuesday weighed in on the Brussels terrorist attacks, commenting on Twitter how "safe" and "beautiful" the Belgian city used to be.

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

Two explosions rocked Zaventem Airport and another ripped through the Maelbeek metro station in Brussels on Tuesday morning.

The prosecutor's office told the Belgian newspaper Le Soir that at least 13 people died and 35 were injured at the airport.

The RTBF reports that 15 people died and that 55 people were injured, 10 of them in serious condition, in the explosion in the metro station, according to the Brussels transport operator STIB.

Belgium's federal prosecutor said the airport explosions were a suicide attack, RTBF reports, though the attackers were not immediately identified.

"I've been talking about this for a long time," Trump said during a Tuesday-morning appearance on "Fox & Friends."

He added: "Brussels was a beautiful city, a beautiful place, with zero crime. And now it's a disaster city."

Trump continued:

We have to be smart in the United States when people come in. We're taking in people without real documentation, we don't know where they're coming from, we don't know ... who they are. You look at them from any standpoint, they could be ISIS, they could be ISIS-related. And we just don't learn. We don't learn.

Trump has previously discussed Brussels on the campaign trail, suggesting that Muslim migration and possibly other factors had led the Belgian capital into a decline.

"I was in Brussels a long time ago, 20 years ago, so beautiful, everything is so beautiful," he said in a January interview. "It's like living in a hellhole right now."

After the Paris and San Bernardino, California, attacks last year, Trump called for the US to stop allowing Muslim tourists and immigrants into the country.

The Paris attacks, which killed 130 people and wounded hundreds more, were thought to have been planned in Belgium, where the police have since conducted raids and made arrests. Salah Abdeslam, the last living suspect thought to be directly involved in the Paris attacks, was arrested in Brussels last week.

Barbara Tasch contributed to this report.