Boris Johnson has said all evidence suggests Bashar al Assad's forces were behind a chemical attack that killed dozens of people in Syria and sparked outrage across the world.

The Foreign Secretary called Tuesday's attack on a rebel-held town in northern Syria a "barbaric act", as Turkey said as many as 100 people were killed and the United Nations held an emergency session in New York.

Damascus has denied responsibility, and Russia, a staunch ally of the Assad regime, said the gas contamination was the result of a leak from a rebel chemical weapons depot hit by government airstrikes.

"According to the objective data of the Russian airspace control, Syrian aviation struck a large terrorist warehouse near Khan Sheikhoun," the Russian defence ministry said.

It said the warehouse contained "toxic substances".


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Image: Dozens of children were hurt in the suspected gas attack

The attack has drawn international condemnation, as footage from the area, Khan Sheikhoun in Idlib province, showed people gasping for air, choking and convulsing in the streets and in overcrowded hospitals.

Others foamed at the mouth as rescue workers hosed down children to wash away any chemicals from their skin and gave them oxygen.

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Turkish President Recep Tayyip Erdogan said 100 people were believed to have died in the attack, and called the Syrian leader a "murderer".

Previously, the British-based Syrian Observatory for Human Rights had put the death toll at 72, including 20 children and 17 women.

"All the evidence I have seen suggests this was the Assad regime who did it in the full knowledge that they were using illegal weapons in a barbaric attack on their own people," said Mr Johnson, as he attended a long-planned conference in Brussels on aid to Syria.

He added he did "not see how a government like that can continue to have any kind of legitimate administration over the people of Syria".

Evidence of Syria attack 'points to Assad'

Later, Mr Johnson told the conference: "Together we should make clear our abhorrence of the regime's tactic of starve or surrender."

"The people of Syria are today paying a price for our collective inaction over the last five years and decisions we took.

"We cannot now undo those mistakes but we can and we must work together to alleviate their suffering, to help Syria's neighbours and to prepare Syria for the moment when peace finally returns."

Iran, like Russia an Assad ally, said it condemns "any use of chemical weapons, regardless of who has used it and who are the victims", and offered assistance to the victims.

The World Health Organisation said victims of the attack appear to show symptoms consistent with exposure to a nerve agent.

"The likelihood of exposure to a chemical attack is amplified by an apparent lack of external injuries reported in cases showing a rapid onset of similar symptoms, including acute respiratory distress as the main cause of death," the UN agency said.

Image: A rescue worker breathes through an oxygen mask after a 'chemical attack' in Idlib

On Wednesday, activists with the Idlib Civil Defence team reported renewed airstrikes in the area.

US President Donald Trump denounced the attack as a "heinous" act that "cannot be ignored by the civilised world" - though he said the previous Obama administration had to take some blame for the situation in Syria.

The UN Secretary-General, Antonio Guterres, said: "The horrific events of yesterday demonstrate unfortunately that war crimes are going on in Syria (and that) international humanitarian law is being violated frequently."

And Pope Francis said he had watched the events in Syria "with horror", deploring what he called an "unacceptable massacre".

Image: A Syrian man in taken by civil defence workers following a suspected toxic gas attack in Khan Sheikhun

The UK, France and the US have proposed a UN Security Council resolution to "condemn in the strongest terms the use of chemical weapons".

The draft calls on the Organisation for the Prohibition of Chemical Weapons to quickly reveal its findings on the attack.

It also urges Mr Guterres to report monthly on whether the Syrian government is co-operating with an international inquiry into the use of chemical weapons.

However, as the Security Council began its emergency meeting, there was little hope the proposed resolution would be adopted: Russia called it "unacceptable", based on "fake" information and "anti-Syrian".

The stance is in line with previous positions held by Russia and China, which have consistently opposed taking action against the Syrian regime.

The attack was one of the worst in Syria's six-year war, and was reminiscent of a 2013 chemical assault that left hundreds dead.