Reuters/Jim Bourg
Republican U.S. presidential candidate Donald Trump reacts during a news conference at the construction site of the Trump International Hotel at the Old Post Office Building in Washington, March 21, 2016.
"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 Molenbeek metro station in Brussels on Tuesday morning.
The prosecutor's office told 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 are in a serious condition, in the explosion in the Molenbeek according to the Brussels transport operator STIB.
Belgium's federal prosecutor said the explosion at the airport was caused by a suicide bomber, RTBF reports. It's unclear who is responsible for the attacks.
"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."
Barbara Tasch contributed to this report.