Russian Federation has called another meeting of the 15-member Security Council on Friday.
One of the officials noted that Syria and Russian Federation relocated their equipment and personnel a year ago - the first and only other time Trump ordered missile strikes on Syria in response to a chemical weapons attack. The operation by the three allies came after a poison gas attack in Syria that killed at least 60 people last week. In April 2017, as he contemplated a strike in Syria, Trump said, "One of the things I think you've noticed about me is: Militarily, I don't like to say where I'm going and what I'm doing".
The US president tweeted this morning, 'Never said when an attack on Syria would take place.
In the past, Trump has condemned others for forecasting military plans, repeatedly blistering President Barack Obama during the 2016 campaign.
A US Navy guided-missile destroyer, the USS Donald Cook, is in the Mediterranean Sea.
British media reported that Prime Minister Theresa May had called her cabinet back from vacation to discuss a response to alleged chemical attacks by Assad in Douma, which Syria and its ally Russian Federation deny.
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Mrs May is reluctant to ask the USA to hold off any action until she has the opportunity to consult MPs, the BBC understands.
During a television interview, French President Emmanuel Macron said on April 12 that he would decide "in due course" whether to strike Syria.
President Bashar al-Assad's government denies being behind any chemical attack. Meanwhile, Sweden has proposed asking the UN Secretary-General, Antonio Guterres, to suggest an impartial mechanism to assign blame for the chemical attack in Syria.
Opposition activists and rescuers say government aircraft dropped bombs filled with toxic chemicals on the town, the last major rebel stronghold near the capital Damascus.
"Japan can not make a judgment on whether Assad's government forces really used chemical weapons", a senior Foreign Ministry official said of the latest attack.
The Associated Press reports that the Organization for the Prohibition of Chemical Weapons (OPCW) will send a fact-finding group to Douma "shortly".
The Syrian state news agency called the threat of a missile strike "reckless escalation".