WASHINGTON — President Trump put Syria and Russia on notice Wednesday morning in a Twitter post, promising that missiles fired at Syria “will be coming, nice and new and ‘smart!’” and telling the Kremlin that it should not partner with a “Gas Killing Animal who kills his people and enjoys it!” After the threat, the president said in a separate tweet that relations between the United States and Russia are worse than ever.
The president appeared to be reacting to reports on Tuesday that the Russian ambassador to Lebanon, Alexander Zasypkin, warned the United States and its allies that any missiles fired at Syria would be shot down. He made those comments in an interview with Al Manar Television.
Mr. Trump’s early morning comments were remarkable in that he is, in a 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.
With American strike intentions so clearly forecast by Mr. Trump, the Syrian government has moved key aircraft to the Russian base near Latakia, and is taking pains to secure important weapons systems, military analysts said.
Pentagon officials said that even if Syrian warplanes manage to elude an American-led strike campaign, the United States and its allies can still seek to so damage Syrian airfields — across the country — that it would hamper Mr. Assad’s ability to use them to launch future chemical weapons attacks.
That kind of damage, though, would require a sustained campaign — likely over a number of days. It was unclear whether the United States, France and other allies involved have made a decision to extend a bombing campaign beyond one night.
But the president’s subsequent tweet struck a different tone. After he warned Russia what it would be up against in Syria, Mr. Trump lamented that relations between the two countries were worse than during the Cold War, a decades-long geopolitical and ideological rivalry between the United States and the Soviet Union when both were armed for, and prepared for, nuclear war.
Russia has blamed the suspected chemical attacks on the Syrian opposition forces. On Wednesday, Maria Zakharova, the Russian foreign ministry spokeswoman, said that if the American missiles were so smart then they should hit “terrorists” and not government targets. She also suggested in a posting on Facebook that the missile attack might destroy evidence of the use of chemical weapons.
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.
The attack on Saturday in the Damascus suburb of Douma has not been confirmed to be the result of a chemical weapon.
Defense Secretary Jim Mattis said on Wednesday that the United States is still assessing the intelligence on the suspected chemical attack, but that military planning was proceeding.
“We stand ready to provide military options if they’re appropriate, as the president determined,” he said.
The World Health Organization said on Wednesday that there were reports of about 500 people in the Damascus suburb of Douma who have symptoms similar to people exposed to toxic chemicals. It said about 70 people had died while taking shelter in basements and 43 of them had signs of being exposed to “highly toxic chemicals.”
Mr. Trump’s comments about poor relations with Russia echoed what the Russian foreign minister, Sergey V. Lavrov, said recently in response to the wave of diplomatic expulsions of Russians from the United States and other countries, according to a Reuters report. The expulsions were a coordinated response to the poisoning in Britain of a former Russian spy and his daughter. Since then, analysts have said the Balkans could become a battleground for a new Cold War.
The tough talk on Russia, when it comes to Syria, is a strikingly different tone for Mr. Trump, who has long pushed for improved relations with the Kremlin. Recently, Mr. Trump praised Mr. Putin for his re-election and even invited him to the White House.
Later on Wednesday morning, Mr. Trump clarified his assessment of the poor relations with Russia in another tweet, blaming the decline in Washington-Moscow ties on the ongoing investigation into Russia’s meddling in the 2016 election.
Russia has been a dominant theme during Mr. Trump’s entire presidency, particularly with the appointment of a special counsel to investigate Russia’s election interference.
The president repeated his frustrations about the ongoing inquiry, which he said was led by Democrats or others who worked for former President Barack Obama.
Earlier this week, the F.B.I. raided the offices and hotel room of Mr. Trump’s personal attorney, Michael D. Cohen, enraging the president, who called it an “attack on our country in a true sense.” Mr. Trump, however, has not used similarly strong language about Russia’s election activities which started as early as 2014.
When it comes to Syria, however, Mr. Trump has blamed Mr. Putin for supporting the Syrian regime. Mr. Trump called the suspected chemical attack a “barbaric act” and suggested Mr. Putin bears some responsibility. “He may, and if he does, it’s going to be very tough, very tough,” Mr. Trump said on Monday.“Everybody’s going to pay a price. He will, everybody will.”
After Mr. Trump’s series of tweets Wednesday morning, Mr. Putin’s spokesman, Dmitry Peskov, said, “We don’t participate in Twitter diplomacy. We advocate serious approaches.” Mr. Peskov’s comments were reported by the Interfax news agency.
Mr. Trump canceled a planned trip to Latin America later this week in order to oversee an American response to Syria, the White House said. And the president met with his military commanders on Monday to discuss options.
But publicly discussing American military plans 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 a few months.
While Mr. Trump’s tweet did not disclose the exact date and time of an American missile strike, Mr. Assad’s allies are lining up to back the Syrian regime.
The top adviser to Iran’s supreme leader said on Wednesday that Tehran would support Damascus against any foreign aggression, Iran’s state television reported.
“Iran backs Syria in its fight against America and the Zionist regime,” Ali Akbar Velayati, the supreme leader’s adviser, told state television during a visit to eastern Ghouta in Syria. Iranian officials call Israel “the Zionist regime.” Mr. Velayati said of the United States, “Their habit is to threaten constantly and the only thing they know how to do is bombing, haven’t Syria and Iran been bombed before?”