Russia, an ally of Syrian President Bashar al-Assad, has responded angrily to the strikes, while Syrian state media called them a "flagrant violation of worldwide law".
Russia's UN Ambassador Vassily Nebenzia told the UN meeting in NY that the U.S. and its allies struck without waiting for an investigation by the Organisation for the Prohibition of Chemical Weapons, branding the attack "hooliganism".
Chemical weapons experts for the Organisation for the Prohibition of Chemical Weapons (OPCW) arrived in Syria to investigate the suspected poison gas attack.
During his meeting with Trump and the Chairman of the Joint Chiefs of Staff Joe Dunford, Mattis pushed for more evidence of the Assad regime's culpability for the attack, to bolster the case for air strikes, The New York Times reported.
"All nations and all people will be harmed if we allow Assad to normalize the use of chemical weapons", she said.
Russia, the main ally of Syrian President Bashar al-Assad, says there is no evidence of a chemical attack in Douma and has warned the United States and its allies against conducting any military strike. Putin and Macron instructed their foreign and defense ministers to maintain close contact to "de-escalate the situation", the Kremlin said.
She added: "It's some of the worst piece of fake news we've yet seen from the Russian Federation propaganda machine".
Nikki Haley said: "When our president draws a red line, our president enforces the red line".
The capture of Douma has clinched a major victory for Assad, crushing what was once a centre of the insurgency near Damascus, and underlines his unassailable position in the war.
Russian Ambassador to the United Nations Vassily Nebenzia (L) and Bolivia's Ambassador to the United Nations Sacha Sergio Llorenty Soliz speak to the media outside Security Council chambers at the U.N. headquarters in NY, U.S., April 12, 2018.
"The US - the possessor of the biggest arsenal of chemical weapons - has no moral right to blame other countries".
Trump administration officials told The Associated Press in February they believed forces loyal to Assad might have been in the process of working on new or more refined chemical weapons.
Any U.S. strike would probably involve the Navy.
He met his national security team later in the day and "no final decision has been made", the White House said in a statement.
Trump has slammed Russian Federation for its partnership with "Gas Killing Animal" Assad, spurring concerns that a United States strike could lead to a conflict with Russian Federation, which has major military facilities at Tartus and Khmeimim and works cheek-by-jowl with Syria forces that could be targeted.