Russia is warning of “grave repercussions” as the U.S. weighs a response to a chemical gas attack in Douma — a rebel-held suburb of Damascus, Syria — that reportedly killed 60 and seriously injured at least 1,000 others; and is additionally requesting an inspection of the site of the attack to prove it either didn’t happen or was staged by Western-backed insurgents.

As President Donald Trump canceled a planned trip to South America this week (Vice President Mike Pence will make the trip instead) reportedly to deal with the growing crisis in the region — for which he said Russia is partially responsible due to their backing of Syrian President Bashar al-Assad — Russia and Iran are supporting Assad as he warns of a response to an air strike on a Syrian military base the three nations blame on Israel.

Moscow’s and Damascus’ claims that a gas attack may never have happened were met with incredulity Monday at a UN Security Council meeting where both the French envoy to the UN and US Ambassador to the UN Nikki Haley took exception to the suggestion.

The French UN envoy, Francois Delattre, said the symptoms of the victims suggested that they had been exposed to “a powerful neurotoxic agent, combined with chlorine to enhance its lethal effect”. Delattre added that only Syrian forces had the means and the motive to make such weapons and carry out such an attempt.

The US ambassador, Nikki Haley, lashed out at Moscow for its unstinting backing of Bashar al-Assad, the Syrian leader. She referred to Moscow as the “Russian regime, whose hands are all covered in the blood of Syrian children”.

Moscow’s ambassador, Vassily Nebenzia, countered the accusations and said that “Russia is being unpardonably threatened”. Nebenzia insisted that Russian investigators had determined that any evidence of a gas attack was merely a “false-flag” operation staged by rebels trained by US special forces.

“There was no chemical weapons attack,” Nebenzia told the council. “Through the relevant channels we already conveyed to the US that armed forces 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.”

Trump promised Monday that once it was determined who carried out the attack in Douma, the US was open to delivering a forceful response over the course of the following few days.

“We’ll be making that decision very quickly, probably by the end of today. But we cannot allow atrocities like that. Cannot allow it,” he told reporters Monday.

The gas attack in Douma gave Assad ultimate control over eastern Ghouta, which had been the biggest rebel-held area near Damascus.