UNITED NATIONS — Infuriating the United States and its allies, Russia on Thursday blocked an American-sponsored resolution at the Security Council to extend the life of a panel investigating who is using chemical weapons in the Syria conflict.
The Russian veto means the panel, which has found that both the Syrian government and Islamic State militants have used chemical poisons in the war, will be dissolved as of Friday.
Ambassador Nikki R. Haley of the United States denounced the veto, saying “it strikes a deep blow,” and accused the Russians of having given a green light to chemical weapons attacks, a war crime.
“The message to anyone listening is clear: In effect, Russia accepts the use of chemical weapons in Syria,” Ms. Haley told the council. Later, addressing the Russians, she said: “The next chemical weapons attack is on your heads.”
Ambassador Vassily Nebenzia of Russia ridiculed the American indignation, accusing the United States and its partners of seeking to preserve a panel with “extremely systemic flaws.”
“There was nothing balanced in the U.S. resolution,” Mr. Nebenzia said. Later he told reporters waiting outside the Security Council chambers that Russia “condemns the use of chemical weapons by anyone” but that the panel’s reporting was “a joke — complete nonsense.”
It was the 10th time Russia had used its veto power as a permanent member of the Security Council to protect the Syrian government, its principal ally in the Middle East.
The veto threw into doubt whether those responsible for chemical weapons use in Syria will ever be held to account. The bitter differences expressed by the United States and Russia, reminiscent of the Cold War, also called into question whether they can cooperate more broadly on ways to end the Syria war, which is nearly seven years old.
Eleven of the 15 members of the Security Council voted in favor of the American resolution, with Russia and Bolivia opposed, and China and Egypt abstaining.
After a short recess, the council reconvened to vote on an alternate resolution drafted by Russia to extend the panel’s mandate, under conditions that the United States and its allies said would have eviscerated its abilities. That resolution received four yes votes — far below the nine needed for passage.
Russia has expressed deep anger over the panel’s Oct. 27 report, which found that the Syrian military had been responsible for a deadly sarin attack on the village of Khan Sheikhoun in northern Syria on April 4.
Panel investigators relied on interviews, photos, videos and analysis of soil samples, including those supplied by the Syrian government from Khan Sheikhoun. They did not go to the attack site because of security concerns.
Calling the report deeply flawed, riddled with inconsistencies and tainted by Western pressure to vilify the Syrian government, the Russians rejected the findings and suggested that Syrian insurgents or Islamic militants might have staged the attack.