At least 100 people have been killed in Syria after a hospital was bombed while treating victims of a chemical nerve gas attack.

The bombardment of the hospital in Idlib in the rebel-held area of northern Syria led to the death of 58, including 11 children, according to the Syrian Observatory for Human Rights.

UK Prime Minister Theresa May condemned the attack. 'We condemn the use of chemical weapons in all circumstances. If proven, this will be further evidence of the barbarism of the Syrian regime,' she said. 

Many others were wounded, and in addition there were suffocation cases and other signs of gas poisoning. 

The Observatory said many people were choking and some had foam and saliva coming out of their mouths - symptoms of gas poisoning. All the children were under the age of eight. 

The White Helmets civil defence group said that one of its own medical centres had been attacked. 

Foreign Secretary Boris Johnson said that if the Assad regime was proven to be responsible, it was a 'war crime' and the president of Syria must be held to account.

The death toll is expected to rise as children and adults succumb to the gas and their subsequent injuries caused by the bombing.

The United Nations is understood to be investigating after the bombardment brought down rubble 'on top of medics as they worked', according to AFP.

François Hollande of France was among the international leaders who condemned the air strikes, and suggested Syria's President Assad was acting with the complicity of his country's allies. France called for an emergency UN Security Council meeting.

A Syrian military source strongly denied the army had used any such weapons.

Previously, the United Nations and the Organisation for the Prohibition of Chemical Weapons have found that various parties in the Syrian war have used chlorine, sulfur mustard gas and sarin.

Additional reporting by Reuters