The Russian envoy also asserted that Russia, Mr. Assad’s biggest ally, had done far more than the West to achieve peace in the Syria conflict. He accused Washington of having adopted “a categorical policy to unleash military force against Syria” and contain Russia.

Mr. Trump’s threats of a strike on Mr. Assad’s forces, the Russian envoy said, were “unworthy of a permanent member of the Security Council.”

Ms. Haley said she was incredulous at Mr. Nebenzya’s defense of the Syrian government and his overall portrayal of events. “I’m in awe of how you say what you say with a straight face,” she told the Russian ambassador.

Ms. Haley called the use of chemical weapons in Syria “a violation of all standards of morality.”

Referring to the Douma assault, Ms. Haley said: “We know who did this. Our allies know who did this. Russia can complain all it wants about fake news, but no one is buying its lies and its cover-ups.”

Her criticism of Russia and Syria was echoed by the envoys of Britain and France, who collectively form the brunt of the Western diplomatic response.

Ambassador Karen Pierce of Britain said her government believed Mr. Assad’s forces had used chemical weapons “consistently, persistently, over the past five years.”