“There have been several allegations of chlorine attacks in Ghouta, in Idlib, and also now recently in Afrin. While we cannot independently verify these allegations, but if confirmed, it is outrageous and should have no impunity,” de Mistura told a UN Security Council meeting on Monday.

An alleged chemical attack on Saturday in the militant-held town of Douma in Eastern Ghouta reportedly left dozens dead.

The remarks by de Mistura came shortly after US State Department spokeswoman Heather Nauert said Washington believed Russia was shielding the Syrian government from accountability for its alleged use of chemical weapons.

Staffan de Mistura reiterated Secretary-General Antonio Guterres’ appeal to those fighting and all concerned “to de-escalate immediately and unconditionally.”

During the UNSC meeting, US Ambassador to the United Nations Nikki Haley said that Washington “will respond” to incident regardless of whether the United Nations Security Council takes action or not.

“We have reached the moment when the world must see justice done,” she said.

“History will record this as the moment when the Security Council either discharged its duty or demonstrated its utter and complete failure to protect the people of Syria…Either way, the United States will respond,” she added.

In response, Russia warned the US of “grave repercussions” if it attacks Syria over claims of a chemical weapons attack.

“There was no chemical weapons attack,” said Russian UN Ambassador Vassily Nebenzia during a UN Security Council meeting held on Monday.

“Through the relevant channels we already conveyed to the US that armed force under mendacious pretext against Syria – where, at the request of the legitimate government of a country, Russian troops have been deployed – could lead to grave repercussions,” he added.

He added that investigators with the global chemical weapons watchdog should travel to Syria as early as Tuesday to investigate accusations concerning the attack.

“Our military, radiological, biological, chemical unit was on site with the alleged chemical accident and it confirmed that there was no chemical substances found on the ground. There were no dead bodies found. There were no poisoned people in the hospitals. The doctors in Douma denied that there were people who came to the hospital claiming that they were under the chemical attack. The Syrian Red (Crescent) that was said to be treating people which were poisoned denied that it was ever doing that today. So what we’re saying — we are requesting the OPCW (Organization for the Prohibition of Chemical Weapons), which said in the person of its director general that they were ready to go to Douma, to do it immediately and to see themselves what happened on the ground,” he added.

At the meeting, Syria’s ambassador to the United Nations Bashar Ja’afari also stressed, “The Syrian Arab Republic stresses once again that it does not possess any chemical weapons of any type, including chlorine, and we condemn once again the use of chemical weapons at any time anywhere and under any circumstance”.

“The Russian Center for Reconciliation in Syria announced today that military experts have carried out investigations in Douma, and these investigations suggest that there are no signs of the use of chemical weapons there. And while treating the sick that are being treated in the hospitals of Douma, Russian doctors have proven that these patients have not been subjected to any chemical substance. So what we are witnessing here is really a Hollywood scene. Thank you, Mr President,” he added.