US intelligence shows a Russian claim that the attack was faked was false, Sanders said.
US Ambassador Nikki Haley said the use of chemical weapons in Syria threatens world peace and that Russian Federation is responsible.
Moscow also said it was calling an emergency session of the United Nations Security Council, where it is a permanent member, to discuss the "aggressive actions" of the U.S. and its allies.
Trump has singled out Russian President Vladimir Putin for his support of Assad, as well as Iran.
The war of words between the USA and Russian Federation over Syria's use of chemical weapons against its own people has escalated.
Russia, which has stonewalled diplomatic efforts at the UN Security Council, has vehemently denied a chemical attack took place and accused the West of seeking an excuse for military action.
Later, a White House briefing on a call between Trump and May said that they "continued their discussion of the need for a joint response to Syria's use of chemical weapons".
U.S. Defense Secretary Jim Mattis has been urging caution in meetings, trying to avoid a strike that would trigger a broader conflict, two U.S. officials said.More news: Sega Announces the Mega Drive Mini for 2018
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Inspectors from the Organisation for the Prohibition of Chemical Weapons are expected to arrive in Syria over the weekend to investigate the reported attack, following an invitation from Damascus.
Since 2015 France has carried out air strikes against Islamic State (IS) militants in Syria as part of allied forces linked to the US -led coalition, conducting about 5 percent of total coalition air missions.
"Our President has not yet made a decision about possible action in Syria".
Mr Putin and Mr Macron instructed their foreign and defence ministers to maintain close contact to "de-escalate the situation", the Kremlin said. "But should the United States and our allies decide to act in Syria, it will be in defense of a principle on which we all agree", Haley told the U.N. Security Council.
"All nations and all people will be harmed if we allow Assad to normalize the use of chemical weapons".
The Kremlin on Saturday condemned Western air strikes on Syria where its armed forces are backing President Bashar al-Assad.
USA allies have offered strong words of support for Washington but no clear military plans have yet emerged.