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".
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".
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