"Now they have to pay the price," Donald Trump writes on Truth Social, partly in capital letters.
The president adds in his post that the Iranian military is “a total mess” and has been “completely defeated.”
He followed up the post a few hours later at a White House press conference, saying the United States "will attack" Iran.
"We are the world's greatest military power, and sometimes you have to use it," he says.
According to Fox News, he told one of its reporters that the US should target Iranian power plants and bridges.
The Iranian military responded to Trump in a statement saying it has so far "proved" that it can respond to American threats and that it will not be intimidated, Al Jazeera reports.
Foreign Ministry spokesman Esmaeil Baqaei said in state media that Iran's military is fully prepared to defend the country.
"Diplomacy and military force do not contradict each other; they are two sides of the same strategy to defend Iran's interests and security," he says.
As recently as Tuesday, Trump said that the parties were in the "final stages" of negotiations to reach a peace agreement.
