Russia blames deadly Syrian gassing on rebels' own chemical arsenal

Russia on Wednesday blamed the poisonous gas contamination that activists say killed at least 83 people — including 25 children — on a leak from a chemical weapons cache hit by Syrian government air strikes.

The alleged gas attack in Idlib province, documented in horrific images that NBC News has not verified, would mark one of the worst incidents of its kind in Syria's six-year civil war.

The U.S. has said the government of Syrian President Bashar Assad was responsible for the incident, which relief agency UOSSM says injured at least 350.

However, Russian defense ministry spokesman Igor Konoshenkov said Tuesday morning's deadly leak came from a rebel "chemical warfare munitions" workshop that had been struck.

"Syrian aviation made a strike on a large terrorist ammunition depot and a concentration of military hardware in the eastern outskirts of Khan Sheikhoun town," he said in a video statement, referring to the rebel-held town in northern Syria where the deaths occurred.

He said "chemical-laden weapons" made by rebels at the site had previously been used by militants in Iraq — an apparent reference to a report last month from the International Committee of the Red Cross that toxic agents had been used in fighting near Mosul.

Russia is a key supporter of Assad, who has been fighting rebels trying to unseat him for more than six years. Konoshenkov also claimed chemical munitions had been used by rebels in Aleppo last year.

Related: Syrian Government Blamed for Third Chemical Attack: Report

Syria's government said it was complying with the 1997 Chemical Weapons Convention, which bans such instruments of war, according to SANA, the Syrian state-run news agency. Instead, the government blamed rebels — that it refers to as "armed terrorist organizations" — for the attack.

"The Syrian Arab republic stresses that all those fabricated allegations will not prevent it from continuing its war on terrorism ... and from working for a political solution to the crisis in Syria," SANA reported.

NATO Secretary-General Jens Stoltenberg on Wednesday condemned the "horrendous attacks" in Idlib. "This is the third report of the use of these barbaric weapons in the last month alone. All those responsible must be held to account," he said in a statement.

The Organization for the Prohibition of Chemical Weapons, which monitors the chemical weapons treaty, said it had set up a fact-finding mission to get to the bottom of Tuesday's attack.

U.S. Secretary of State Rex Tillerson, said the attack clearly used banned chemical weapons.

Tillerson noted that the attack was the third time such weapons are alleged to have been used this month, charging: "It is clear that this is how Bashar al-Assad operates: with brutal, unabashed barbarism.

"Those who defend and support him, including Russia and Iran, should have no illusions about Assad or his intentions," Tillerson said in a statement. "Anyone who uses chemical weapons to attack his own people shows a fundamental disregard for human decency and must be held accountable."