Syria 'chemical attack': US weighs up military response
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.
Western leaders appear to be gearing up for air strikes, but Russian figures have said they will retaliate against any strikes on their ally Syria.
On Thursday, Kremlin spokesman Dmitry Peskov said it was "very necessary to avoid any steps which could increase tensions in Syria".
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At the UN Security Council, Russia and the US blocked each other's proposals for investigating claims of a chemical attack, as they disagreed over who should have the right to apportion blame.
The international Organisation for the Prohibition of Chemical Weapons (OPCW) has said it will go to Douma "shortly", but it is unclear when it would arrive and how much evidence of any chemical attack might remain.
The town is being evacuated through a humanitarian corridor, under terms of an agreement between Russia and the rebels.
The main leaders of the rebel group Jaish al-Islam have left the town, and the group's weapons have been surrendered, according to UK-based monitor the Syrian Observatory for Human Rights.
Russian claims that the town had been taken over by Syrian forces and a government flag raised on a building could not be independently verified.
Major General Yury Yevtushenko said it was a "significant event in the history of Syria", and meant the whole Eastern Ghouta area was in government control.
What has the US said?
"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".
Then on Thursday, he tweeted that any attack could be "very soon or not so soon at all".
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.
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".
What happened in Douma?
Activists, rescue workers and medics say dozens of people died when government aircraft dropped bombs filled with toxic chemicals on 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.
The Syrian-American Medical Society (Sams), which operates in rebel-held areas, said more than 500 people had been treated for symptoms "indicative of exposure to a chemical agent".
On Wednesday, the UN's World Health Organization demanded access to verify reports from its partners, which include Sams, that 70 people had died - including 43 who showed "symptoms consistent with exposure to highly toxic chemicals".
What is Russia's position?
It has described the reports of the chemical attack as a "provocation" designed to justify Western intervention against its ally, and accused militant rebels of fabricating it.
It said on Wednesday that samples taken from the site had not revealed any chemical substances.
Senior Russian figures have warned of a Russian response to a US attack.
On Wednesday Alexander Zasypkin, Moscow's ambassador to Lebanon, repeated a warning by the head of the military that missiles would be shot down and their launch sites targeted if they threatened the lives of Russian personnel.
And Russian foreign ministry spokeswoman Maria Zakharova asked whether the aim of Western strikes might be "to quickly remove the traces of the provocation... [so] international inspectors will have nothing to look for in terms of evidence".
Addressing new ambassadors in Moscow, President Vladimir Putin said he hoped common sense would prevail and that the situation would stabilise.
Mr Putin said Russia would "keep all its international obligations in full".