The UN Secretary General has called a suspected chemical weapons attack that left more than 70 people dead in Syria a “moment of truth” amid anger over the failure to prevent repeated atrocities in the six-year conflict.

Antonio Guterres told a conference co-hosted by the UK in Brussels that the massacre in the rebel-held town of Khan Sheikhoun must be investigated.

“The horrific events of yesterday demonstrate that unfortunately war crimes are going on in Syria, that international humanitarian law remains being violated frequently,” he said.

Mr Guterres said he was confident that UN Security Council meeting due later in the day would “live up to its responsibilities” after Russia and China repeatedly vetoed sanctions on Bashar al-Assad’s regime over chemical weapons.

The Syrian government denied deploying toxic gas in the latest incident in Idlib, with Russia claiming a “terrorist” weapons development facility caused the deaths when it was hit by legitimate air strikes – an allegation rejected by opposition groups.

Doctors said victims started to choke, vomit and faint with foam coming out of their mouths, showing symptoms of possible sarin gas exposure.

The World Health Organisation (WHO) has not named the toxin involved as investigations continued but said the evidence was “consistent” with reactions to a nerve agent.

Victims of alleged chemical attack in Idlib, Syria

The UN agency said at least 70 people were killed and hundreds more affected in Khan Skeikhoun, adding: “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.

“Some cases appear to show additional signs consistent with exposure to organophosphorus chemicals, a category of chemicals that includes nerve agents.”

Chemical weapons were first reportedly used in Syria in 2012, WHO said, and have since occurred with “disturbing frequency” despite a 2013 agreement aiming to reduce the regime’s stockpiles.

Boris Johnson said he had seen “absolutely nothing to suggest” that anyone but Assad’s forces could have been responsible for the massacre, which is being investigated as a potential war crime by the UN.

"All the evidence I've seen suggests that 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,” the Foreign Secretary told reporters in Brussels.

"You cannot go on with a regime that's willing to use illegal weapons against its own people, a regime that's killed hundreds of thousands of its own people.

“What's needed now is a political process to get rid of that regime and give the people of Syria a chance.”

A Syrian child receives treatment following a suspected gas attack in Khan Sheikhoun, a rebel-held town in the northwestern Syrian Idlib province, on 4 April (AFP/Getty)

Mr Johnson said it was “impossible” to imagine continuing power for the Assad regime after its latest alleged war crime, adding: “This is a government that has absolutely no compassion for its own people that has put itself beyond the pale.“

He and his French counterpart, Jean-Marc Ayrault, renewed calls for Assad to step down as part of a lasting political solution in Syria, just days after suggestions from the US Secretary of State and UN Ambassador that his removal was no longer a priority.

Donald Trump’s spokesman also blamed the Syrian government for the “reprehensible” attack – as well as the Obama administration’s “inaction”.

Germany’s foreign minister, Sigmar Gabriel, called on Russia to endorse a planned UN Security Council resolution aiming to “bring to justice those who are responsible”.

Also appearing at the Brussels conference was the Lebanese Prime Minister, Saad Hariri, who said the world had failed to prevent atrocities like the Khan Sheikhoun attack.

“The world should not be shocked because it's letting such a regime do what it is doing,” he added.

“What should shock us is the increase of children dying and that the whole world is watching.”

He told reporters that “everyone is coming to Brussels to make a statement and the regime made its statement in Syria.”

Destruction at a hospital that was bombed while treating victims of a suspected chemical attack in Khan Sheikhoun, Idlib, on 4 April (AFP/Getty Images)

Syrian activists and rebels were also voicing their frustration over global inaction over the repeated use of chemical weapons in Syria and other atrocities they say have been overlooked as the world focuses on Isis.

“It is unacceptable that six years into Syria’s crisis, civilians continue to be slaughtered with impunity,” said Assaad al-Achi, from the opposition NGO Baytna Syria.

“Syrians are depending on Europe to show real moral courage and help stop the killing. We hope Europe will not let us down.”

Successive rounds of UN-brokered peace talks in Geneva have failed, while Russia and China have vetoed a resolution to refer the Syrian regime to the International Criminal Court and governments including the UK have been defeated in parliamentary votes on military action against Assad.

Mr al-Achi called on the EU to take “active measures” to enforce a Syrian ceasefire, rather than placing responsibility on Russia, Iran and other supporters of the Assad regime holding rival peace talks in Astana.

Syrian opposition representatives are among delegates at the Brussels Conference on Supporting the Future of Syria and the Region, which is being held over two days in the Belgian capital.

Delegates were expected to pledge huge amounts of aid for humanitarian assistance inside Syria, as well as for more than five million refugees who have fled the country.

Mr Johnson was criticised after praising Lebanon and Turkey for setting a “moral example to the world” by taking in millions of asylum seekers, when the UK is falling short of its own targets and has scrapped a programme to resettle child refugees.

Mr Hariri said Lebanon has been overwhelmed by the arrival of around 1.5 million Syrians and “cannot sustain this issue anymore”, adding: “The international community has to do something.”