Officials in Russia, which has provided military and political support to Mr. Assad throughout the Syrian conflict, also rejected the accusations.
“I am not aware of any information about a threat that chemical weapons can be used,” Dmitry S. Peskov, the Kremlin spokesman, said on Tuesday. “Certainly, we consider such threats to the legitimate leadership of the Syrian Arab Republic unacceptable.”
A senior Russian lawmaker accused the United States of using the declaration about chemical weapons to plan an attack on Syria.
“Preparations for a new cynical and unprecedented provocation are underway,” Frants Klintsevich, first deputy chairman of the defense and security committee in the upper chamber of the Russian Parliament, told the state-owned news agency RIA Novosti.
The United States and other world powers have accused Mr. Assad’s forces of repeatedly using chemical weapons to subdue rebels seeking to topple his government. Chemical attacks killed more than 1,000 people near Damascus in 2013 and dozens more in northern Syria in April of this year.
Mr. Trump has taken a different approach to the use of chemical weapons in Syria than his predecessor, President Barack Obama, did. After the 2013 attack, Mr. Obama declined to strike the Syrian government, despite having declared the use of chemical weapons a “red line.” Instead, he agreed to a deal, proposed by Russia, for the Syrian government to dispose of its chemical weapons stockpiles and manufacturing capabilities.
But American officials suspect that Syria kept some of its chemical weapons capabilities. After the attack in April, Mr. Trump ordered 59 Tomahawk cruise missiles to be fired at the air base from which the attack originated.