LONDON -- Groundwork is being laid for an internationally coordinated military response to the Syrian government's alleged use of chemical weapons on a rebel-held neighborhood in Syria.
Later in a White House briefing, Press Secretary Sarah Sanders appeared to walk back the president's words, saying that military options still were being discussed.
"The president has a number of options at his disposal," she said. "We haven't laid out any specific actions we plan to take."
Trump's tweet received sharp caution from the Russians. Several hours after it was posted, Russian President Vladimir Putin, who's supporting Syrian President Bashar al Assad, said "the state of things in the world cannot but provoke concern."
"Nevertheless we hope that common sense will prevail in the end," he said, in a meeting with new Russian ambassadors.
The Foreign Ministry Spokeswoman Maria Zakharova wrote on Facebook: "Smart rockets ought to fly towards terrorists and not at a lawful government."
The speaker of Russia's parliament said that the "whole world is horrified by" the president's tweet.
After a Russian ambassador in Lebanon said Russia would shoot down any U.S. missiles and launch sites, a senior military official giving a briefing on Wednesday said that Russia was "following the situation closely," adding that the U.S. coalition "ought to be occupying itself with the restoration of the destroyed city" of Raqqa instead of considering air strikes against the Syrian government.
Meanwhile in the U.K., Prime Minister Theresa May is convening her cabinet on Thursday morning to discuss backing the U.S. with a military response over the alleged chemical attack. She is not seeking parliamentary approval for military action but the agreement of the cabinet, which is expected to back a response of non-lethal military assistance to the U.S. against Syrian targets.
In France, President Emmanuel Macron is expected to clarify his position later on Thursday in a television interview –- only his third since taking office last year.
He has said that any French strikes would target the Syrian government's chemical facilities and has spoken at length with Trump since Sunday, after the first reports of a possible chemical attack in Syria emerged.
In Syria, there are reports that forces allied with the Syrian government have been relocating from various military sites and bases in anticipation of a possible attack.
A U.K.-based Syrian monitor says that troops have been emptying airports and bases, while opposition fighters have told Sky News Arabia that government-allied forces have been retreating from some positions and moving equipment.
In the wider region, a Russian news agency is reporting that the Russian navy is conducting military exercises close to the coast of Syria. Starting on Wednesday and continuing Thursday, the exercises mean that airspace above Russian vessels in the Mediterranean will be closed due to live fire, according to Interfax.
British media reported that U.K. submarines in the Mediterranean have been ordered to move within range of Syrian targets ahead of May's decision expected later on Thursday.
Meanwhile commercial flights appear to be avoiding Syrian airspace. The Lebanese carrier Middle East Airlines has temporarily halted all flights over Syrian airspace, and Kuwait Airlines is suspending all flights landing in Beirut's Rafik Hariri Airport.