UNITED NATIONS — Britain called an emergency session of the United Nations Security Council on Wednesday to formally accuse Russia of trying to murder a former Russian spy and his daughter on British soil with a military-grade nerve agent.

The March 4 attack in the British cathedral town of Salisbury was “indiscriminate and reckless,” Ambassador Jonathan Allen said, part of what he suggested was a pattern of Russian misbehavior that had become a threat to international peace and security.

He received unequivocal support from his American counterpart, Nikki R. Haley, who, unlike her boss, President Trump, has bluntly rebuked Russia on a range of topics.

“Russia is responsible for the attack on two people in the United Kingdom using a military-grade nerve agent,” Ms. Haley said, calling the poisoning “an atrocious crime.”