The images of the victims — including many children — foaming at the mouth and convulsing as they struggle to breathe after a suspected chemical weapons strike is a shocking reminder to the world that the long war in Syria is far from over.

The attack Tuesday in the northwestern province of Idlib is believed to have killed at least 100 people, according to groups who monitor the war. More than 100 others were injured.

The early morning air raid believed to have been carried out by Syria's military or its ally, Russia, also highlights the continued failure of the international community to stop the horrors of the conflict, which has raged for more than six years and left as many as 500,000 people dead.

Dr. Shajul Islam posted video to YouTube that appears to show patients from Tuesday's suspected chemical attack being treated at a medical centre in Idlib province, Syria. (screengrab/CBC) "The hospital is now overwhelmed with patients," said Dr. Shajul Islam, a physician who posted video Tuesday that appears to show wounded from the attack being treated at a medical centre in Idlib province."No one is doing anything to stop these gas attacks."

Chemical weapons experts say early analysis of the attack on the rebel-held town of Khan Sheikhoun points to the use of sarin gas, a nerve agent. The common symptoms — choking, convulsions and foaming at the mouth — were seen in Islam's video as well as others purporting to show the victims of the strike this morning.

It is a clear escalation in the long-running violence, even in a conflict where chemical weapons have been used many times.

The United Nations and the global chemical weapons watchdog, the Organization for the Prohibition of Chemical Weapons, concluded last year that the Syrian government used chlorine against civilians at least three times in 2014 and 2015. The same report also determined that ISIS used the blister agent sulphur mustard (commonly known as mustard gas) at least once.

From Damascus, the response has always been the same: a flat-out denial that the Assad regime has ever used chemical weapons, even in the face of facts that state otherwise.

Two men are treated after the suspected gas attack Tuesday in the town of Khan Sheikhoun in rebel-held Idlib, Syria. (Ammar Abdullah/Reuters)

Witnesses in Khan Sheikhoun report seeing warplanes in the air just as Tuesday's strikes were carried out. Syrian opposition and rebel forces — including ISIS — do not have the ability to hit targets from the air. Syria's military and the Russian air force, stationed in Syria for more than a year and a half, do.

Assad blamed a 2013 chemical weapons strike on rebel fighters, even though the United States came forward with evidence that showed Syrian government forces were behind the deadliest use of chemical weapons during the war. The attack on the Damascus suburb of Ghouta left more than a thousand people dead.

The use of sarin gas in Ghouta crossed the "red line" drawn by Barack Obama. But instead of acting on the threat of military action, the former U.S. president agreed to a deal brokered by the Russians that saw Assad give up his chemical weapons stockpile.

Chemical weapons experts say Tuesday's attack appears to show that the Syrian regime did not hand over its entire stockpile to be destroyed.

Syrian civil defence volunteers, known as the White Helmets, try to extinguish fire reportedly caused by airstrikes in the northwestern city of Idlib on March 24, 2017. (Omar Haj Kadour/AFP/Getty Images)

The Ghouta incident "led to the disarming of the chemical weapons arsenal possessed by Syria — which, unfortunately, has not been completed yet in actuality," said Dany Shoham, a former chemical weapons specialist for Israel's military, the Israel Defence Forces.

Analysts estimate that Assad's forces held onto about 10 per cent of the regime's chemical weapons.

'This is clearly a war crime'

The Syrian president is clearly emboldened by recent gains, most notably recapturing territory lost to rebels in Aleppo. His grip on power in Syria looks solid, largely because of the unflinching support of key allies such as Russia and Iran.

But a recent change in tone in Washington probably boosted Assad's confidence, as well.

Under the Trump administration, American policy on Syria is no longer focused on forcing Assad from power, a departure from Obama's long-held stance that the Syrian president cannot be a part of any political solution to end years of war.

"You pick and choose your battles. And when we're looking at this, it's about changing up priorities — and our priority is no longer to sit there and focus on getting Assad out," U.S. ambassador to the UN Nikki Haley said recently.

Just days later, U.S. Secretary of State Rex Tillerson said Assad's fate "will be decided by the Syrian people."

The American shift puts the U.S. at odds with its European allies and Canada, which maintain that Assad has to go.

And that could complicate a global response to the alleged chemical attack in Idlib province.

It demonstrates once again that the regime will stop at nothing to remain in power, even the most heinous use of weapons imaginable. - Matthew Rycroft, U.K. ambassador to the UN

"This is clearly a war crime," the U.K.'s ambassador to the UN, Matthew Rycroft, said to reporters Tuesday. "It demonstrates once again that the regime will stop at nothing to remain in power, even the most heinous use of weapons imaginable."

Canada's minister of foreign affairs, Chrystia Freeland, said in a statement Tuesday: "Not all the facts are yet available, but this deplorable incident is consistent with the actions of a regime that has brutally and repeatedly used chemical weapons against its own people."

The UN Security Council is expected to hold an emergency session Wednesday to discuss the Khan Sheikhoun raid, but Russia could once again use its veto to protect president Assad.

The White House blamed the Syrian government for Tuesday's attack, calling it "reprehensible."

Despite that, President Trump's shift in policy on Syria has few expecting that the U.S. will establish a new "red line" on the use of chemical weapons in the conflict.

That has many battle-hardened medical workers and emergency responders in Syria worried that this latest attack will not be the last.