WASHINGTON — President Trump put Syria and Russia on notice Wednesday morning, promising that missiles fired at Syria “will be coming, nice and new and ‘smart!’” — while also telling the Kremlin that it should not partner with a “Gas Killing Animal who kills his people and enjoys it!”

Mr. Trump’s early morning comments were remarkable in that he is, in some way, telegraphing the United States’ response to a suspected chemical weapons attack in Syria, which is something he had previously criticized other leaders for doing. Mr. Trump has said publicly that sharing military plans could give enemies information they could use to their advantage.

On Tuesday, the Russian ambassador to Lebanon, Alexander Zasypkin, warned the United States and its allies that any missiles fired at Syria would be shot down, according to a Reuters report.

Mr. Trump has been critical of Russia and its president, Vladimir V. Putin, for supporting the Syrian regime, led by Bashar al-Assad, believed to be behind the suspected chemical weapons attack on April 7 that has left dozens dead.

Mr. Trump canceled a planned trip to Latin America later this week in order to oversee an American response to Syria, according to a White House statement. The president met with his military commanders on Monday to discuss options.

But publicly discussing American military plans, as Mr. Trump did in his Twitter post on Wednesday when he said missiles “will be coming,” is in contrast to how he has said he would conduct himself as commander in chief.

During tensions with North Korea in April of 2017, he said in an interview on “Fox & Friends” that he would not say whether he would order a strike if the rogue nation continued conducting missile tests.

“I don’t want to telegraph what I am doing or what I am thinking,” Mr. Trump said. “I am not like other administrations, where they say, ‘We are going to do this in four weeks.’ It doesn’t work that way. We’ll see what happens.”

That was the kind of message that Mr. Trump repeatedly delivered as a presidential candidate, mocking former President Barack Obama for giving adversaries too much information by setting timelines for withdrawal from combat zones.

And, indeed, while he has not set a public withdrawal deadline for American forces in Syria the way Mr. Obama did for other combat zones, just last week Mr. Trump set a private one that quickly became public when he told military commanders that ideally he wanted to pull troops out of Syria within six months.