Russia denied on Wednesday that Syrian President Bashar al-Assad was to blame for a poison gas attack and said it would continue to back him, setting the Kremlin on course for its biggest diplomatic collision yet with Donald Trump's White House.

Western countries, including the United States, blamed Assad's armed forces for a chemical attack that choked scores of people to death in the town of Khan Sheikhoun in a rebel-held area of northern Syria on Tuesday.

Washington said it believed the deaths were caused by sarin nerve gas dropped by Syrian aircraft. But Moscow offered an alternative explanation that would shield Assad: that the poison gas belonged to rebels and had leaked from an insurgent weapons depot hit by Syrian bombs.

'Everyone saw the plane … bombing with gas.' — Hasan Haj Ali, Free Idlib Army

Video uploaded to social media showed civilians sprawled on the ground, some in convulsions, others lifeless. Rescue workers hosed down the limp bodies of small children, trying to wash away chemicals. People wailed and pounded on the chests of victims.

The World Health Organization said the symptoms were consistent with exposure to a nerve agent.

The United States, Britain and France proposed a draft UN Security Council resolution that would have demanded an investigation.

The Security Council held an emergency meeting on Wednesday about the attack, during which Russia and Syria again rejected the allegations — blaming the release of gas on the rebels while accusing other council members of relying on misinformation to attempt a regime change in Damascus. 

'Have you even checked what you wrote?' — Deputy Russian UN Ambassador Vladimir Safronkov

Moscow's representative repeated the Kremlin's earlier claim that the gas was released when a conventional airstrike hit a warehouse where rebels were secretly storing their own chemical weapons. 

Russia, which is among the countries with veto power over UN resolutions, said the draft proposal by the "anti-Damascus camp" was based on "falsified reports … taken at face value." 

"Have you even checked what you wrote?" deputy Russian UN ambassador Vladimir Safronkov asked. "This draft was prepared in a hasty way."

Safronkov added that former president Barack Obama's threat of military action if a "red line" was crossed and chemical weapons were used in Syria had provoked such attacks.

"That decision served as a starting point for future provocations by terrorists," he said, remarks that echoed those of President Donald Trump, who faulted Obama for having failed to enforce the red line four years ago.

Russia has blocked all previous resolutions that would harm Assad, most recently in February.

Lines crossed

Trump accused Assad's government of going "beyond a red line" with a poison gas attack on civilians and said his attitude toward Syria and Assad had changed, but gave no indication of how he would respond.

Trump said the attack "crosses many, many lines," an allusion to his predecessor Barack Obama's threat to topple Assad with air strikes if he used such weapons. His accusations against Assad put him directly at odds with Moscow, the Syrian's president principal backer.

"I will tell you, what happened yesterday is unacceptable to me," Trump told reporters at a news conference with Jordan's King Abdullah on Wednesday.

"And I will tell you, it's already happened that my attitude toward Syria and Assad has changed very much," though when asked at an earlier meeting whether he was formulating a new policy on Syria, Trump said: "You'll see."

Vice-President Mike Pence, when asked whether it was time to renew the call for Assad to be ousted and safe zones be established, told Fox News: "But let me be clear, all options are on the table," without elaborating.

'Damning indictment of Assad'

On a call from the Brussels Conference on Supporting the Future of Syria and the Region, Canada's Foreign Minister Chrystia Freeland urged the UN Security Council to support a resolution to confirm responsibility for the latest chemical attack.

"If confirmed, this new use of chemical weapons is a damning indictment of Assad," she said. "And I want to be very clear that Canada considers this to be an absolutely reprehensible attack on civilians, including children."

Freeland said the American role in resolving the Syrian crisis is a "truth, universally acknowledged." She pointed to the work of the U.S. ambassador to the United Nations.

"[Nikki Haley] has been very active, very strong and very outspoken in the debate today at the Security Council. Personally, I would like to commend the work that she's doing today and the strong position she has taken."

Hasan Haj Ali, commander of the Free Idlib Army rebel group, earlier called Russian's blame of the rebels a "lie" and said rebels did not have the capability to produce nerve gas.

"Everyone saw the plane while it was bombing with gas," he told Reuters from northwestern Syria. "Likewise, all the civilians in the area know that there are no military positions there, or places for the manufacture [of weapons]."

The Britain-based Syrian Observatory for Human Rights said Wednesday the death toll had risen to 86, and those killed in Tuesday's attack included 30 children and 20 women.

'Heinous actions'

The incident is the first time Washington has accused Assad of using sarin since 2013, when hundreds of people died in an attack on a Damascus suburb. At that time, Washington said Assad had crossed a "red line" set by Obama.

Obama threatened an air campaign to topple Assad but called it off at the last minute after the Syrian leader agreed to give up his chemical arsenal under a deal brokered by Moscow, a decision which Trump has long said proved Obama's weakness.

TURKEY SYRIA RUSSIA PROTEST

Protesters shout slogans against Russia for its alleged role in a chemical attack in Idlib province, Syria, near the Russian consulate in Istanbul on Tuesday. (Sedat Suna/EPA)

The new incident means Trump is faced with same dilemma that faced his predecessor: whether to openly challenge Moscow and risk deep involvement in a Middle East war by seeking to punish Assad for using banned weapons, or compromise and accept the Syrian leader remaining in power at the risk of looking weak.

Quietly dropping demands

Trump's response to a diplomatic confrontation with Moscow will be closely watched at home because of accusations by his political opponents that he is too supportive of Russian President Vladimir Putin.

MIDEAST-CRISIS/SYRIA-ATTACK

Men ride past a hazard sign at a site hit by an airstrike on Tuesday in the town of Khan Sheikhoun in rebel-held Idlib. The sign reads, 'Danger, unexploded ammunition.' (Ammar Abdullah/Reuters)

He has previously said the United States and Russia should work more closely in Syria to fight against Islamic State.

U.S. intelligence agencies say Russia intervened in the U.S. presidential election last year through computer hacking to help Trump defeat Hillary Clinton. The FBI and two congressional committees are investigating whether figures from the Trump campaign colluded with Moscow, which the White House denies.

The chemical attack in Idlib province, one of the last major strongholds of rebels that have fought since 2011 to topple Assad, complicates diplomatic efforts to end a war that has killed hundreds of thousands of people and driven half of Syrians from their homes.

Over the past several months Western countries, including the United States, had been quietly dropping their demands that Assad leave power in any deal to end the war, accepting that the rebels no longer had the capability to topple him by force.

The use of banned chemical weapons would make it harder for the international community to sign off on any peace deal that does not remove him.

British Foreign Secretary Boris Johnson, who two months ago shifted his country's policy by saying Assad could be allowed to run for re-election, said on Wednesday that he must go.

"This is a barbaric regime that has made it impossible for us to imagine them continuing to be an authority over the people of Syria after this conflict is over." 

With files from CBC News and The Associated Press