Donald Trump declared a "bomb went off" in New York City, and suggested it was linked to terrorism, before officials had released any details on the blast which ripped through the Chelsea neighbourhood of Manhattan injuring at least 29 people.
Hillary Clinton also said she had been briefed "about the bombing", but refused to comment further until more "facts" became clear - and suggested her presidential rival should have done the same.
Amid confusion over the cause of the explosion,, the Republican candidate appears to have jumped the gun.
He made the announcement minutes after stepping off his plane during a rally at an airport hangar in Colorado Springs, Colorado.
"I must tell you that just before I got off the plane a bomb went off in New York and nobody knows what's going on," Mr Trump said.
He continued: "But boy we are living in a time - we better get very tough, folks. We better get very, very tough. It's a terrible thing that's going on in our world, in our country and we are going to get tough and smart and vigilant."
Ms Clinton said she had been briefed "about the bombings in New York and New Jersey and the attack in Minnesota". In New Jersey, a pipe bomb was detonated along the route of a 5k run for US Marines, while in St Cloud, Minnesota, police said multiple people were injured in a gun and knife attack at a shopping mall on Saturday evening. The suspect was reported to be dead.
Speaking on her campaign jet, she said she would respond further "when we actually know the facts".
Asked specifically about her reaction to Mr Trump describing the blast as a "bomb" and alluding to it as an attack, she said: "It's important to know the facts about any incident like this.
"I think it's always wiser to wait until you have information before making conclusions."
The White House said President Barack Obama has been told of the explosion and will be updated as additional information becomes available.
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