UNITED NATIONS — The United States on Tuesday accused Syria’s government of a chlorine gas attack on civilians in the same rebel enclave hit more than four years ago by the deadliest known chemical assault in the Syrian war.

In sharp denunciations from Ambassador Nikki R. Haley at the United Nations and Secretary of State Rex W. Tillerson in Paris, the Americans also rebuked Russia for what they called its failure to stop such assaults, which under international law are war crimes. The Russians called the American accusations “baseless.”

The chlorine attack happened Monday in Eastern Ghouta, an insurgent redoubt near Syria’s capital, Damascus, that has defied President Bashar al-Assad’s forces since the war began nearly seven years ago. The Syrian Observatory for Human Rights, a monitoring group, said at least 13 people were hurt by rockets loaded with chlorine gas. Others said 21 were sickened.

Eastern Ghouta was the target on Aug. 21, 2013, of an attack using lethal sarin nerve agent, which by some estimates killed 1,400 people. That assault stunned the world and led President Barack Obama, who had called the use of chemical weapons a “red line,” to threaten military retaliation on Mr. Assad’s forces.