The American ambassador, Nikki R. Haley, accused Russia of putting “its friends in the Assad regime” before global security.
“It’s a sad day for the Security Council when member states make excuses for other members killing their own people,” she said in the council chambers after the vote.
The resolution, proposed by Britain and France months ago and endorsed by the United States last week, would have imposed sanctions on a handful of Syrian military officials and entities for having dropped chlorine-filled barrel bombs on opposition-held areas on at least three occasions in 2014 and 2015, according to a United Nations panel.
On Friday Ms. Haley said after a Security Council meeting on Syria that such attacks were “barbaric.” Chlorine is banned as a weapon under an international treaty that Mr. Assad’s government signed in 2013.
The arguments and vote over the resolution were important because they provided new insight into how President Trump, who has made clear his intent to improve ties with Russia, would deal with the Kremlin over the Syria war. Russia is Mr. Assad’s most important foreign ally.
The conflict over the resolution was in sharp contrast to a Russian-American consensus on the need to contain Syria’s use of chemical weapons. After a sarin gas attack on a suburb of Damascus in August 2013, Moscow and Washington struck a deal to force Mr. Assad to sign the chemical weapons treaty and dismantle his stockpile of the poisonous munitions under international supervision.
The Syrian government, though, violated the deal, according to a United Nations panel set up by the Security Council, known as the Joint Investigative Mechanism. It found that the government had used chemical weapons at least three times.
Russia helped to create the panel but questioned its findings when it implicated the Syrian government. The panel also found that Islamic State militants in Syria used mustard gas in August 2015.
Moscow made clear last week that it would defeat the draft measure to impose sanctions on the Syrian government, calling it unbalanced. The Russian veto signaled how far Russia was willing to go to shield its ally in Damascus.
President Vladimir V. Putin of Russia reinforced his opposition on Tuesday, adding that any Security Council penalties on the Syrian government would complicate diplomatic efforts underway in Geneva aimed at halting the war.
“As for sanctions against the Syrian leadership, I think the move is totally inappropriate now,” he told a news conference while visiting Kyrgyzstan. “It does not help, would not help the negotiation process. It would only hurt or undermine confidence during the process.”
Human Rights Watch concluded in a recent report that the Syrian military had not only violated its promises not to use chemical weapons but had systematically dropped chlorine bombs in the final weeks of the battle to take the northern city of Aleppo last fall.
Mr. Trump repeatedly has expressed admiration for Mr. Putin of Russia and said he wanted to strike a deal with him to stop the war in Syria and focus on fighting terrorism. But disagreements within Mr. Trump’s administration appear to have complicated that goal.
Ms. Haley has taken a hard line against Russia. She condemned what she called Russia’s “aggressive actions” in eastern Ukraine, vowed to maintain sanctions related to the Russia’s annexation of Crimea, and in her Senate confirmation hearing, went as far as saying that Russia was guilty of war crimes in Syria.
Her comments on Russia, often directly contradicting her boss, echo the talking points of the previous administration of Barack Obama, but they also reflect the concerns of Republicans in Congress, who distrust the Kremlin.
Ms. Haley was in Washington on Monday for meetings at the White House. A former governor of South Carolina, she has by her own admission limited foreign policy experience.
She has so far kept her comments limited to a handful of foreign policy issues that plainly deliver political dividends at home. She has maintained a tough line on Russia and Iran, pledged to defend Israel, and promised more oversight into how American funding for the United Nations is spent.
She has said nothing about the Trump administration’s travel ban on refugees and visa applicants from seven predominantly Muslim countries, which the United Nations secretary general, António Guterres, has criticized.
Ms. Haley, an American of Indian descent who grew up in a small South Carolina town, also has been silent on the attack on two Indian engineers in Kansas last week, which was suspected to be a hate crime and which threatens to cloud Indian-American relations.
