WASHINGTON -- U.S. President Donald Trump on Thursday put off a final decision on possible military strikes against Syria after tweeting earlier that they could happen "very soon or not so soon at all." The White House said he would consult further with allies.

Defence Secretary Jim Mattis warned such an attack carried the risk of spinning out of control, suggesting caution ahead of a decision on how to respond to an attack against civilians last weekend that U.S. 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 emphasized that Syrian use of chemical weapons should not be tolerated. And he insisted it remains U.S. 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 U.N.-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.

Mattis' remarks at a House Armed Services Committee hearing followed a series of Trump tweets this week that initially indicated he was committed to bombing Syria but later suggested he was awaiting further advice and assessment. Trump wrote in a Thursday morning tweet that an attack could happen "very soon or not so soon at all."

Later 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 airstrikes, 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 U.S., France and Britain have been in extensive consultations about launching a military strike as early as the end of this week, U.S. officials have said. A joint military operation, possibly with France rather than the U.S. in the lead, could send a message of international unity about enforcing the prohibitions on chemical weapons.

Macron said Thursday that France has proof that the Syrian government launched chlorine gas attacks and said France would not tolerate "regimes that think everything is permitted."

After May met with her Cabinet, a spokesperson issued a statement saying it is highly likely that Syria's President Bashar Assad was responsible for Saturday's attack that killed dozens outside Damascus. The Cabinet agreed on the need to "take action" to deter further chemical weapons use by Assad, but added that May would continue to consult with allies to co-ordinate an international response.

Mattis said that although the United States has no hard proof, he believes the Syrian government was responsible for Saturday's attack. Initial reports indicated the use of chlorine gas, possibly in addition to the nerve agent Sarin. Trump's ambassador to the United Nations, Nikki Haley, told NBC News on Thursday the administration has "enough proof" of the chemical attack but was still considering its response.

The Organization for the Prohibition of Chemical Weapons, based in the Netherlands, announced it was sending a fact-finding team to the site of the attack outside Damascus, and it was due to arrive Saturday. It was not clear whether the presence of the investigators could affect the timing of any U.S. military action.

At the House hearing, Democrats grilled Mattis on the wisdom and legality of Trump ordering an attack on Syria without explicit authorization from Congress. Mattis argued it would be justified as an act of self-defence, with 2,000 U.S. ground troops in Syria; he insisted he could not talk about military plans because an attack "is not yet in the offing."

Mattis said he personally believes Syria is guilty of an "inexcusable" use of chemical weapons, while noting that the international fact-finding team would likely fall short of determining who was responsible.

Asked about the risks of U.S. military retaliation, Mattis cited two concerns, starting with avoiding civilian casualties.

"On a strategic level, it's how do we keep this from escalating out of control, if you get my drift on that," he said.

Rep. Adam Smith, the ranking Democrat on the House Armed Services Committee, said he sees no legal justification for a U.S. strike in Syria, absent explicit authorization by Congress. More broadly, he doubted the wisdom of bombing.

"Until we have a more long-term strategy, until we have some idea where we're going in Syria and the Middle East, it seems unwise, to me, to start launching missiles," said Smith, D-Wash. "We need to know where that's going, what the purpose of it is before we take that act."

At stake in Syria is the potential for confrontation, if not outright conflict, between the U.S. and Russia, former Cold War foes whose relations have deteriorated in recent years over Moscow's intervention in Ukraine, its interference in the 2016 U.S. presidential election and its support for Syrian President Bashar Assad.

Russian lawmakers have warned the United States that Moscow would view an airstrike on Syria as a war crime and that it could trigger a direct U.S-Russian military clash. Russia's ambassador to Lebanon said any missiles fired at Syria would be shot down and the launching sites targeted -- a stark warning of a potential major confrontation.

At the House hearing, Rep. Tulsi Gabbard, D-Hawaii, disputed Trump's legal authority to act without congressional authority and suggested a U.S. strike would lead to war with Russia.

"I'm not ready to speculate that that would happen," Mattis said.