At least 58 people have been reported killed in what witnesses claim was a chemical weapons attack in northern Syria, as world leaders discuss ending the conflict in Brussels.

Doctors in the rebel-held town of Khan Sheikhoun, in Idlib province, said victims started to choke, vomit and faint with foam coming out of their mouths.

The UK-based Syrian Observatory for Human Rights said at least 11 children and several women were among dozens killed, with reports of "suffocation" following the bombings.

It put the death toll at 58 but warned the number could rise as several victims remained in a serious condition.

Dr Shajul Islam said the hospital where he worked was filling up with patients, posting footage online of victims he said showed symptoms of exposure to sarin.

"We have samples," he wrote. "Will anyone care? Who will stop it?"

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

Rescue workers from the White Helmets group were seen wearing gas masks to protect themselves while treating people lying amid rubble on the ground.

They reported at least 20 civilians among those affected, saying chlorine gas may have been responsible.

Activists posted video online claiming to show the moment the bombs struck, alleging they were dropped by Syrian air force helicopters as clouds of smoke rose into the air.

It came little over a week after another alleged chemical attack hit a hospital in the town of Latamneh, just 14 miles away.

Médecins Sans Frontières (MSF), which supports the facility, said a bomb dropped by a helicopter struck the entrance on 25 March.

Syrian government blamed for Aleppo chemical attack

“Immediately after the impact, patients and staff reported suffering severe respiratory symptoms and burning of mucous membranes – symptoms consistent with an attack using chemicals,” a spokesperson said.

The hospital’s orthopaedic surgeon was among two people killed in the attack, leaving just two orthopaedic surgeons for a population of around 120,000 people, and the clinic was put out of service for three days.

​Bashar al-Assad's forces have been accused of repeatedly using the nerve agent during the six-year civil war, including in a massacre in Eastern Ghouta that almost sparked military intervention by the UK in 2013.

Damascus has denied the allegations, while Isis is also alleged to have used chemical weapons including chlorine and mustard gas.

Idlib province, in north-western Syria, is a key battleground between regime forces and rebels including Islamist factions linked to al-Qaeda.

It was the destination for rebel fighters evacuated from eastern Aleppo earlier this year, and is widely expected to be Assad's next major target.

In pictures: Children play underground in Syria Show all 8 1 /8 In pictures: Children play underground in Syria In pictures: Children play underground in Syria A child plays in the ball pit at the 'Land of Childhood,' an underground playground in a besieged town in Syria Unicef In pictures: Children play underground in Syria Abdulaziz, 10, plays in the 'Land of Childhood' underground playground in Syria Unicef In pictures: Children play underground in Syria Boys look through a show window inside the tunnel that provides a safe passage for children between the two basements that form the 'Land of Childhood' in a besieged town in Syria Unicef In pictures: Children play underground in Syria Children play at the underground 'Land of Childhood' in a besieged town in Syria Unicef In pictures: Children play underground in Syria Two girls play at the 'Land of Childhood' underground playground in a besieged town in Syria Unicef In pictures: Children play underground in Syria Children on the ride on a ferris wheel at the 'Land of Childhood', an underground playground in a besieged town in Syria Unicef In pictures: Children play underground in Syria Seven-year-old Massa at the "Land of Childhood" underground playground in a besieged town in Syria Unicef In pictures: Children play underground in Syria Children wait in line to buy sweets at the 'Land of Childhood,' an underground playground in a besieged town in Syria Unicef

Khan Sheikhoun sits near the frontline between regime and opposition held territory, on a key motorway linking the city of Hama with rebel strongholds to the north.

The alleged chemical attack came as world leaders gathered in Brussels for a conference organised by the EU and UN on "supporting the future of Syria".

Successive rounds of peace talks in Geneva have so far failed to produce a lasting ceasefire in the Syrian conflict, which shows no sign of ending in its seventh year of bloodshed.

Calls for Assad’s removal – once the stated priority for the US, UK, Turkey and other Nato members – has waned in priority since the rise of Isis and other extremist groups.