Doctors who treated children affected by a chemical weapons attack in northern Syria on Tuesday have spoken out about the devastating scenes they witnessed.

At least 72 civilians, including 20 children, were killed in what is feared to be the worst chemical weapons attack in Syria in almost four years, according to the Syrian Observatory for Human Rights.

Hospitals treating victims of a suspected toxic gas were themselves targeted in a series of air strikes following the atrocity in a rebel-held town in Idlib province, where activists claimed up to 100 people died.

At a hospital in Idlib, about 30 miles away from Khan Sheikhoun, the town that was hit, Doctor Mizar Hassani said he treated around 30 patients, among them four children.

“They just woke up to find everything in their life changed,” Dr Hassani told ABC News via Skype, adding that the children kept crying.

“When we receive them they are crying … we will give them our medication, our treatment. When they wake up they are crying. I cannot continue ... their father, mother died. What will we do for them?”

Dr Hassani explained that when the victims of the attack were drowsy and agitated and had lesions and runny noses on arrival at the hospital — symptoms consistent with a gas attack. He added that the children he treated survived and were doing better now.

Abdulhai Tennari, who also treated victims, said the attack happened at 6am local time, and that six hours later children started to arrive at his hospital.

“Children arrived dead. They were found under the rubble. There are children who don’t have their parents,” he said.

“We don’t know where their parents are. We treated them and are looking for their parents. Their parents might be looking for them.”

Dr Tennari said many of the injured died immediately, most of them choking on gas, while others died on their way to the hospital where some survived and were treated.

UN war crimes investigators have launched a probe into both rounds of bombing, saying the use of chemical weapons or deliberate targeting of medical facilities “would amount to war crimes and serious violations of human rights law”.

Responding to the incident, Sonia Khush, Save the Children's Syria Director, warned that following the “brutal” events of Tuesday, the international community must act to prevent events from “escalating into the situation we saw in East Aleppo".

“Doctors at a health clinic run by our partner Syria Relief told us they received three children under six years old today," said Ms Kush.

"They were struggling to breathe and barely conscious, with running noses and contracted pupils – doctors say these symptoms are consistent with the use of nerve agents such as Sarin.

Aleppo before the Syrian Civil War Show all 12 1 /12 Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A man crosses a street in Aleppo, December 12, 2009 Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A vendor sits inside an antique shop in al-Jdeideh neighbourhood, in the Old City of Aleppo, December 12, 2009 Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A view shows part of Aleppo's historic citadel, overlooking Aleppo city, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A view shows part of Aleppo's historic citadel, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War Visitors walk inside Aleppo's Umayyad mosque, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War People walk inside the Khan al-Shounah market, in the Old City of Aleppo, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A man walks past shops in al-Jdeideh neighbourhood, in the Old City of Aleppo, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War People walk along an alley in al-Jdeideh neighbourhood, in the Old City of Aleppo, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War Visitors tour Aleppo's historic citadel, Syria December 11, 2009 Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A general view shows the Old City of Aleppo as seen from Aleppo's historic citadel, Syria December 11, 2009 Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War People walk near Aleppo's Bab al-Faraj Clock Tower, Syria October 6, 2010 Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A man stands inside Aleppo's historic citadel, overlooking Aleppo city, Syria December 11, 2009 Reuters

“If a banned chemical substance is confirmed, this would be in clear violation of international law and a worrying indication that not all chemical weapons have been destroyed in Syria as UNSCR 2118 reached in September 2013 demanded.

“Idlib is home to some two million civilians, including many people who were displaced by the conflict in neighbouring Aleppo and other areas. Tomorrow’s meeting of Foreign Ministers and the UN Secretary General in Brussels must address the threat of ongoing assault and brutality faced by people in Idlib.