"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," WHO said in a statement Wednesday.

The WHO said it was "alarmed by serious reports of the use of highly toxic chemicals in an attack in Khan Shaykhun, southern rural Idlib, Syria."

On Tuesday Syrian opposition health minister, Firas Jundi, said more than 100 civilians had been killed and 500 others, mostly children, injured in a chlorine gas attack carried out by regime warplanes in the town of Khan Shaykun, Idlib province.

Chlorine is nerve agent.

The WHO said that according to local Health Cluster partners on the ground treating patients, at least 70 people died in the attack and hundreds more were hurt.

"Doctors in Idlib are reporting that dozens of patients suffering from breathing difficulties and suffocation have been admitted to hospitals in the governorate for urgent medical attention, many of them women and children," WHO said.

Reports first emerged of the use of chemical weapons agents in Syria in 2012 and have since occurred with disturbing frequency, including repeated allegations of chlorine use in and around Aleppo last year, especially in September-December 2016. This latest reported incident is the most horrific since Ghouta in August 2013, WHO said.

Noting that amid the limited capacity of local hospitals and after the initial attack, Al Rahma Hospital received patients and was made temporarily nonfunctional when it was damaged, WHO said, "Emergency rooms and intensive care units in Idlib are overwhelmed and reporting shortages in medicines required to treat injured patients. Many patients have been referred to hospitals in southern Turkey."

"WHO is shipping additional medicines from Turkey and is ready to provide more life-saving supplies and ambulances as needed. WHO experts in Turkey are communicating with health workers in Idlib to provide around-the-clock guidance on diagnosis and appropriate treatment of patients," WHO said.

Tuesday’s attack came one day after regime aircraft reportedly carried out a similar chlorine gas attack on Idlib’s town of Al-Habit, injuring or hurting dozens.

Last year, a UN-appointed investigation panel found that chemical weapons were used by regime forces and opposition fighters in 2014 and 2015. However, no actionable steps were taken.

Anadolu Agency