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.”