Defense Secretary Mattis, however, indicated that evidence of what happened was still being studied.
With the UN Security Council failing thus far to find a diplomatic solution, Secretary General Antonio Guterres warned Wednesday that time was running out.
Mattis said options would be discussed with Trump at a meeting of his National Security Council on Thursday afternoon. However, Russia has said its experts have found no trace of "chlorine or any other chemical substance used against civilians".
The Russian government will compile a list of US nationals who will be forbidden to enter Russia, within the framework of a draft law on new sanctions, leader of the Russian LDPR party Vladimir Zhirinovsky said Friday.
"They are the guarantors of law and order in the town", RIA news agency quoted Russia's defence ministry as saying.
Russian Ambassador to the United Nations Vassily Nebenzia (left) and Bolivia's Ambassador to the United Nations Sacha Sergio Llorenty Soliz speak at the USA headquarters in New York, April 12, 2018.
President Assad launched a bloody offensive on Eastern Ghouta along with Russian military assistance.
"We must avoid a nuclear conflict", Pushkov said.
"The intelligence provided certainly paints a different picture", Sanders said.
"We invited the new president to visit our Cossack homestead, since it was thought that a Putin-Trump meeting would take place in the Russian president's hometown", he added.
"Insulting the President of Russian Federation is unacceptable and inadmissible", Antonov said in his statement responding to the US-led military action.
Trump's tweets were more belligerent. "Get ready Russian Federation, because they will be coming, nice and new and 'smart!' You shouldn't be partners with a Gas Killing Animal who kills his people and enjoys it!"
Konashenkov released statements he said came from medics at Douma's hospital, saying a group of people toting video cameras entered the hospital, shouting that its patients were struck with chemical weapons, dousing them with water and causing panic.
U.K. Prime Minister Theresa May said the attack was "not about intervening in a civil war".
British leader May said in London that the West had tried "every possible" diplomatic means to stop Assad from using chemical weapons. "We are acting together with our American and French allies".
Damascus agreed to hand over its chemical arsenal in 2013, narrowly avoiding American and French air strikes in retaliation for a suspected sarin attack.
Administration officials and diplomats say foreign governments have learned to discount many of Trump's tweets, particularly those clearly aimed at spinning up his political base or goading foreign adversaries.
Syrians who had been bracing for USA strikes for three nights in a row said the mood in the capital, Damascus, was palpably relaxing, with residents increasingly shrugging off the threat as the days passed.
Here's who Russian Federation has blamed this week for the alleged chemical attack that killed dozens, including many children. Russian Federation repeated its warnings against an escalation.
Trump made his latest statement in a tweet Thursday morning.
But Nebenzia said the goal was to oust President Bashar al-Assad from power.
But she says, "No final decision has been made".
"France will not allow any escalation that could harm stability in the region", he added.
Trump further slammed Russian Federation for its continued support of Syrian President Bashar al-Assad in Wednesday's tweet by saying it "shouldn't be partners with a Gas Killing Animal who kills his people and enjoys it!" The ministry additionally pointed to previous alleged use of chemicals by the rebels in fighting with Syrian government troops.
He also called the Syrian leader a "gas killing animal".