President Donald Trump has put off a final decision on possible military strikes against Syria after tweeting earlier they could happen "very soon or not so soon at all."
The White House said on Thursday he would consult further with allies while Defence Secretary Jim Mattis warned such an attack carried the risk of spinning out of control.
Mattis's remark suggested caution ahead of a decision on how to respond to an attack against civilians last weekend that US officials are increasingly certain involved the use of banned chemical weapons.
British officials said up to 75 people were killed.
The White House press secretary, Sarah Huckabee Sanders, said in a brief statement after Trump met with Mattis and other members of his National Security Council: "No final decision has been made. We are continuing to assess intelligence and are engaged in conversations with our partners and allies."
Sanders said Trump would speak later with French President Emmanuel Macron and British Prime Minister Theresa May.
Although Mattis noted that military action carried risks, he also emphasised that Syrian use of chemical weapons should not be tolerated. And he insisted it remains US policy not to be involved directly in Syria's civil war.
"Our strategy remains the same as a year ago," he said. "It is to drive this to a UN-brokered peace but, at the same time, keep our foot on the neck of ISIS until we suffocate it," referring to the Islamic State extremist group.
Trump wrote in a Thursday morning tweet that an attack could happen "very soon or not so soon at all."
But later on Thursday he was noncommittal. "We're looking very, very seriously, very closely at the whole situation," he told reporters.
Mattis said options would be discussed with Trump at a meeting of his National Security Council on Thursday afternoon. That meant air strikes, possibly in tandem with France and other allies that have expressed outrage at the alleged Syrian chemical attack, could be launched within hours of a presidential decision.
The US, France and Britain have been in extensive consultations about launching a military strike as early as the end of this week, US officials have said.
A joint military operation, possibly with France rather than the US in the lead, could send a message of international unity about enforcing the prohibitions on chemical weapons.
Macron said on Thursday that France had proof that the Syrian government launched chlorine gas attacks and said France would not tolerate "regimes that think everything is permitted."
The British Cabinet agreed on the need to "take action" to deter further chemical weapons use by Assad, but added that Prime Minister Theresa May would continue to consult with allies to coordinate an international response.
Mattis said although the US had no hard proof, he believed the Syrian government was responsible for Saturday's attack.
The Organisation for the Prohibition of Chemical Weapons announced it was sending a fact-finding team to the site of the attack outside Damascus.
Russian lawmakers have warned the US that Moscow would view an air strike on Syria as a war crime and it could trigger a direct US-Russian military clash.