UNITED NATIONS — Holding photographs of dead Syrian children after a chemical bomb attack, the United States ambassador to the United Nations warned on Wednesday that her country might take unilateral action if the Security Council failed to respond to the latest atrocity in the Syria war.

Facing her first serious Syria showdown at the Security Council, the ambassador, Nikki R. Haley, also used her remarks at an emergency session to blame Russia for blocking a robust response to the attack on Tuesday on a northern Syrian town, which has incited widespread condemnation. The death toll was reported to exceed 100.

The United States, France and Britain have accused the Syrian government of responsibility and bitterly criticized Russia — Syria’s main ally in the six-year-old war — for objecting to a resolution they drafted condemning the attack.