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The US says "all options are on the table" in response to a suspected chemical weapons attack in Syria, as Western leaders continue to weigh up military action.
White House spokeswoman Sarah Sanders told reporters that a final decision on military strikes had not been taken.
But she said the US holds Russia and Syria responsible for the incident.
The US National Security Council is to meet on Thursday, while UK PM Theresa May has called a cabinet meeting.
Activists, rescue workers and medics say dozens of people died in the formerly rebel-held town of Douma on Saturday.
But President Bashar al-Assad's government - which receives military backing from Russia - denies being behind any chemical attack.
Tens of thousands of people have reportedly left Douma through a humanitarian corridor and Russia's defence ministry says the Syrian flag is flying there. This claim has not been independently verified.
"The president has a number of options at his disposal and a number remain on the table," Ms Sanders said during a briefing on Wednesday evening.
"We haven't laid out any specific actions we plan to take," she added.
The comments appear to row back on President Donald Trump's earlier warning to Russia that it should "get ready" for missile strikes in Syria.
"Get ready Russia, because they will be coming, nice and new and 'smart!'" Mr Trump said in an early morning tweet on Wednesday.
He also called the Syrian leader a "gas killing animal".
Skip Twitter post by @realDonaldTrump
Donald J. Trump
Russia vows to shoot down any and all missiles fired at Syria. Get ready Russia, because they will be coming, nice and new and “smart!” You shouldn’t be partners with a Gas Killing Animal who kills his people and enjoys it!
10:57 AM - Apr 11, 2018
126K people are talking about this
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End of Twitter post by @realDonaldTrump
What happens next?
The US, UK and France have agreed to work together and are believed to be preparing for a military strike in response to the alleged chemical attack last Saturday.
Defence Secretary James Mattis said the US was still assessing the chemical attack, but added that the military stood ready "to provide military options if they are appropriate as the president determines".
The White House said the president had not laid down a specific timetable for when military strikes may take place. But a senior source has told the BBC's North America editor Jon Sopel that any military action is likely to happen at the weekend.
Are we heading for a third world war?
A US Navy guided-missile destroyer, the USS Donald Cook, is in the Mediterranean Sea.
In the UK, sources told the BBC that Prime Minister Theresa May looked ready to join military action in Syria without seeking parliamentary consent first, as she is understood to be reluctant to ask the US to hold off any action while she consults MPs.
French President Emmanuel Macron will reportedly decide on a response in the coming days.
He has said any strikes would target the Syrian government's "chemical capabilities".
Other key figures have been showing signs that they anticipate strikes to happen soon. Senior US military sources say President Assad's forces have already moved some aircraft in an attempt to protect them from attacks. The New York Times reports Syrian planes have been taken to a Russian base near Latakia on the country's coast, and that some Russian personnel and equipment have also been moved.
Airlines have also taken note of the situation, and no planes are visible over Syria on the commercial flight-tracking website FlightRadar24.
Meanwhile, UN Secretary General Antonio Guterres has warned of the need to avoid the situation "spiralling out of control".