The president consulted other world leaders about holding Mr. Assad accountable for the atrocity Sunday that killed dozens in Douma, a rebel-held suburb of Damascus. According to activist groups, helicopters dropped barrel bombs filled with toxic gas on the town situated on the outskirts of the capital Damascus, killing dozens of civilians and wounding scores more.
Trump planned two meetings with senior national security aides Monday on Syria, in addition to a previously scheduled late-afternoon White House conference with leaders of USA military commands around the world. Vice President Mike Pence will attend the 8th Summit of the Americas in his place, the White House said in a press briefing Tuesday.
Speaking in the White House, Mr Trump said: "It was an atrocious attack, it was terrible, you don't see things like that - as bad as the news is around the world, you just don't see those images".
"The President has been clear".
"And so, working with our allies and our partners from North Atlantic Treaty Organisation to Qatar and elsewhere, we are going to address this issue", Mattis said in brief remarks to reporters as he began a meeting with the emir of Qatar. "I'm not going to get into specifics on intel matters and things that we may or may not do".
However, Nikki Haley, the U.S. ambassador to the UN, insisted America "will respond" whether the UN Security Council acts or not.
Israel is strongly condemning the Syrian government's suspected chemical weapons attack on its own citizens.
Mr Trump did not give a time frame for any retaliatory action, but said the United States could not stand by as such atrocities take place because "we are able to stop it".
British Prime Minister Theresa May, who has called the alleged attack "barbaric", is also set to discuss the situation with Trump on Monday, a Downing Street spokesperson told CNN.
"Russia will inevitably veto the USA resolution criticising Assad, and Washington will use this to justify military strikes", he said. With the help of Russian Federation and Iran, Assad and his forces have just won the battle for the eastern suburbs of Damascus after a bloody, months-long siege.
The US president and his French counterpart Emmanuel Macron agreed in a phone call that chemical weapons had been used in Douma.
Israel does not typically comment on its operations in Syria, and it is unclear whether the missile attack was linked to the alleged use of chemical weapons.
"Russia must abandon its blocking attitude at the UN Security Council, with regards to the investigation of the use of chemical weapons in Syria".
U.N. Secretary-General Antonio Guterres urged the council in a statement Tuesday to "redouble its efforts to agree upon a dedicated mechanism for accountability".
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At a meeting of the United Nations Security Council in New York, Russia's envoy Vassily Nebenzia insisted there was "no chemical weapons attack", despite reports 70 people were killed and 500 injured in the city of Douma on Saturday.
"Russia is being unpardonably threatened".
"History will record this as the moment when the Security Council either discharged its duty or demonstrated its utter and complete failure to protect the people of Syria", Haley declared.
Trump is the 45th President of the United States.
Trump said Monday, "We are getting some very good clarity", regarding who was responsible, a task that has been hard throughout the Syrian war and the source of conflict among the many global players involved.
Adding to the tensions, Iran, a strong ally of Syrian President Bashar Assad, threatened to respond to an airstrike on a Syrian military base on Monday that the Syrian government, Russian Federation and Iran blamed on Israel. Moscow has cautioned the US not to launch a military attack.
Nebenzia rejected a USA call for the world body to set up a probe to identify the perpetrators of what rescuers called a "poisonous chlorine gas attack" late Saturday in the enclave of Douma, near Damascus.
She said: "It is truly horrific to think of victims, families sheltering underground when the chlorine found them".
CNN has not been able to independently verify the authenticity of the images.
Large numbers of people have also crossed into government-held territory rather than leaving along with rebels.
The Syrian conflict has developed into a complex proxy war, which in recent years has included Israeli strikes on Iran-linked targets.
"If it's Russian Federation, if it's Syria, if it's Iran, if it's all of them together, we'll figure it out and we'll know the answers quite soon", Trump said.
And any new action raises the risk of open conflict with Russian Federation which, like Iran, has deployed forces to defend Assad's regime. The opposition's Syrian Civil Defense rescue organization said the victims appeared to have suffocated.