The Trump administration accused Russia and China of “outrageous and indefensible” action Tuesday after they vetoed a U.N. Security Council resolution that would have imposed new sanctions on Syria for using chemical weapons against its own citizens.

In a sharply worded speech after the vote, U.S. Ambassador Nikki Haley said the message the council was sending to the world was that “if you are allies with Russia and China, they will cover the backs of their friends who use chemical weapons to kill their own people.”

Her comments marked a rare administration criticism of Russia, which President Trump has said could be a partner in counterterrorism operations in Syria, and of the Syrian government’s behavior in that country’s five-year civil war.

Russian envoy Vladi­mir Safronkov called Haley’s statement “outrageous” and said that “God shall judge” attempts by the West to discredit the legitimate Syrian government.

The United States sponsored the resolution, along with Britain and France. It followed the October conclusion of a joint investigation by the United Nations and the Organization for the Prohibition of Chemical Weapons that the Syrian government had dropped munitions containing chlorine on at least three occasions in 2014 and 2015.

The OPCW concluded after the alleged attacks that they had taken place, but it had no mandate to assess responsibility. That led the Security Council, with Russian and Chinese backing, to establish the joint investigation to identify the perpetrators.

In a report issued in October, investigators concluded that the Syrian government had dropped chlorine-filled munitions on the three dates in question. The investigation also concluded that the Islamic State had used mustard gas on at least one occasion.

The Tuesday resolution called for travel and economic sanctions against several Syrian air force and intelligence officers linked to the attacks by investigators, along with asset freezes of several Syrian companies and government-linked organizations. It also established a mechanism to monitor compliance.

A single veto from one of the 15-nation council’s five permanent members — Russia, China, the United States, Britain and France — can kill a resolution. Bolivia, one of 10 nonpermanent, rotating members, also voted against Tuesday’s measure.

In denouncing the resolution, Safronkov suggested that evidence was uncorroborated and came from “suspicious eyewitness accounts . . . armed opponents, sympathetic [nongovernmental organizations], media and also the so-called Friends of Syria.”

The latter is an international group, made up largely of U.S. allies, set up in 2012 in response to Russian and Chinese vetoes of previous U.N. resolutions on ­Syria.

Russia, Safronkov said, saw “no convincing evidence on the basis of which any sort of allegations could be made.”

The United States and Russia have been on opposing sides of Syria’s civil war since its outset. While the United States and European and regional allies have supported armed fighters and political opponents against Syrian President Bashar al-Assad, Russia has been Assad’s primary foreign backer.

In 2012, President Barack Obama said that he did not intend to intervene in the Syrian war but also that Assad’s deployment of Syria’s known chemical arsenal could change his calculation. When chemical attacks were reported in the summer of 2013, Obama first said he would use U.S. and allied air power in Syria, but he later backed down when Congress refused to authorize such attacks.

The United States and Russia then forged an agreement — to which Assad acceded under Russian pressure — for the removal and destruction of Syria’s stockpile of mustard gas and nerve agents, a task that was completed in the summer of 2014. Syria also signed the international Chemical Weapons Convention.

But the war continued, including Russia’s decision to supplement Assad’s airstrikes with its own warplanes starting in the fall of 2015.

While chlorine is technically not listed as a chemical weapon, the Security Council in early 2015 passed a resolution, supported by Russia, condemning the use of any toxic chemicals as weapons of war and saying that those responsible would be held accountable.

But Russia, with support from China, continued to veto resolutions that were specifically directed at Syria.

President Trump, who has suggested a possible coalition with Russia against the Islamic State in Syria, has given less attention to the civil war there and has indicated that he may cut back assistance to the armed opposition.

At the United Nations, however, Haley has condemned Russian actions in both Ukraine and Syria.

On Tuesday, a day after meetings at the White House, she said it was “sad day” when Security Council members “start making excuses” for perpetrators of chemical attacks. In vetoing the resolution, she said, Russia and China had “ignored the facts” and “put their friends in the Assad regime ahead of our global security.”