Eleven children and nearly 50 adults have been killed in an alleged chemical weapons attack carried out by the Syrian regime against a rebel-held town, according to activists and a monitoring group.

The Syrian Observatory for Human Rights said 58 people had been killed in the town of Khan Sheikhun in Idlib province by the attack early on Tuesday and that most of the casualties were civilians.

A hospital was then bombed as it struggled to treat the victims of the attack.

One video from the scene showed the bodies of nine young children laid out in a row, with their small forms covered in blankets. Other footage showed victims with white foam around their mouths, a common sign of exposure to chemical weapons.

“We have no doubt that this is a sarin attack,” said Dr Shajul Islam, a British medic working in a hospital in Idlib that received some of the patients from Khan Sheikhun.

The Syrian opposition accused the Assad regime of carrying out the attack and demanded that the UN Security Council hold an emergency session to investigate.

"The National Coalition demands the Security Council convene an emergency session..., open an immediate investigation and take the necessary measures to ensure the officials, perpetrators and supporters are held accountable," said the National Coalition, an opposition umbrella group.

“Failure to do so will be understood as a message of blessing to the regime for its actions.”

There was no immediate response from the Assad regime, which has regularly used chemical weapons during the six-year Syrian war but consistently denied doing so.

This is the third suspected chemical attack in Syria this week and the deadliest since chemical weapons were used near Palmyra in December.

Hours after the initial attack, a clinic in the town of Khan Sheikhun was hit by a rocket as doctors tried to treat the injured. The blast brought rubble down on top of medics, according to AFP, but it was not clear if anyone was killed in the attack.

The levels of violence in Syria have fallen since a Russian-backed ceasefire came into force in December but fighting continues across the country and the opposition accuses the regime of breaching the ceasefire agreement.

Dr Islam said that many of the wounded were arriving at hospital with large pupils that were not responsive to light, a potential sign of chemical poisoning.

He said he was treating three children. Two of them were unconscious while a third, a newborn baby, was only lightly affected. The baby arrived at the hospital alone and medics suspected its parents had been killed in the attack, Dr Islam said.