"Parliament must be consulted on this", he said.
The British leader's office said Cabinet ministers "agreed on the need to take action to alleviate humanitarian distress and to deter the further use of chemical weapons by the Assad regime". British officials said up to 75 people were killed. "We are continuing to assess intelligence and are engaged in conversations with our partners and allies".
Pictures and video footage of the aftermath of another chemical weapons attack blamed on Syrian dictator Bashar Assad are hard to take.
Mattis addressed a hearing of a House of Representatives committee on Thursday.
Later Thursday he was noncommittal.
Later, he told reporters a decision will "be made fairly soon".
"France will shoulder its responsibility to end an intolerable threat to our collective security", he said. USA action that could bring Americans into conflict with Russian and Iranian forces could have disastrous consequences. The statement made no specific reference to military action. This is in contrast to an incident one year ago in which the US government had video and other evidence of certain aspects of an actual attack by Syrian aircraft, which involved the use of sarin gas.
"It is a very, very delicate circumstance, and we've got to make this judgment on a very careful, very deliberate, very well thought-through basis, knowing exactly ... how strong the evidence is", Brexit Secretary David Davis said ahead of Thursday's cabinet.
The U.S. has obtained blood and urine samples from a suspected chemical attack in Syria that have tested positive for both an unidentified nerve agent and chlorine gas exposure, according to a report published Thursday. That then deterred the US administration of Barack Obama from similar action.
Asked whether the USA military was ready to conduct an attack in Syria if ordered, Mattis replied, "We stand ready to provide military options if they're appropriate, as the president determined".More news: Google introduces wireless Android Auto for Pixel, Nexus phones
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The government said it is "highly likely" that Assad is responsible for the Douma attack, with ministers agreeing "it was vital that the use of chemical weapons did not go unchallenged".
Asked about the risks of US military retaliation, Mattis cited two concerns, starting with avoiding civilian casualties.
"We can not risk an escalation even further than it's gone already".
Trump did not detail what a strike on Syria would look like, or whether these would be US missiles. More broadly, he doubted the wisdom of bombing.
"The situation in the world is getting more chaotic", he said. "We need to know where that's going, what the goal of it is before we take that act".
Moisi stressed the "risk of escalation" of the conflict amid increasing concerns about a U.S. -Russia proxy war.
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.
Half a world away in New York, Russia's United Nations ambassador warned the priority in Syria was to avert US-led strikes that could lead to a unsafe confrontation between the world's two preeminent nuclear powers.
"I'm not ready to speculate that that would happen", Mr. Mattis said.
At the House hearing, Rep. Tulsi Gabbard, D-Hawaii, disputed Trump's legal authority to act without congressional authority and suggested a US strike would lead to war with Russian Federation.