"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.
Diplomats said the United States was pushing for a vote on Tuesday on its draft resolution setting up a Syria gas attacks inquiry, but Russia could veto the measure.
Trump slammed the "horrible attack" and vowed to respond "forcefully."
"We're going to make a decision tonight or very shortly thereafter," he told reporters.
Rescuers and medics in Douma say more than 40 people died after the suspected poison gas attack in the last rebel-held pocket of the one-time opposition stronghold of Eastern Ghouta.
Access to the area, which has faced weeks of regime bombardment, is limited and AFP has not independently verified the accounts.
In April last year, Trump launched a cruise missile strike against a Syrian air base after a previous chemical weapons attack UN monitors later pinned on Bashar al-Assad's regime.
'Very dangerous'
The punitive raid did not draw US forces any deeper into Syria's civil war, nor did it dissuade Syria or its Russian and Iranian backers from pursuing their campaign against rebel groups.
And any new action raises the risk of open conflict with Russia which, like Iran, has deployed forces to defend Assad's regime.
Nebenzia, reflecting earlier warnings from President Vladimir Putin's Kremlin, said: "From what we hear now, I am afraid they are looking for a military option, which is very, very dangerous."
Earlier, Trump was asked whether Putin bore some of the responsibility for the latest attack. "Everybody is going to pay a price. He will. Everybody will," the American president warned.
Washington's main European allies stood by reports blaming Assad.