GAZIANTEP, Turkey (Reuters) - Wearing chemical suits and gasmasks, Syrian doctors rush to a house where white smoke wafts over a group of people choking and coughing, some calling out for help.

It is a training exercise but the scenario is all too real for many of the doctors, who treated victims of a chemical attack three months ago and suffered symptoms themselves after being contaminated by a deadly nerve agent.

Around 100 people were killed in the sarin gas attack on the opposition-held northern Syrian town of Khan Sheikhoun on April 4, the international chemical weapons watchdog OPCW said. Two hundred people needed treatment, including medical staff.

The United States and Western allies blamed the Syrian government for the attack - an accusation President Bashar al-Assad dismissed as “fabrication” - and launched cruise missile strikes on a Syrian air force base in response.

The doctors hope the week-long course in southern Turkey organized by the World Health Organization, will leave them better prepared and better protected for any future attack.

It is the most intensive training on chemical warfare provided to Syrian medical staff, who have also treated patients for chlorine gas attacks during Syria’s brutal six-year war.

Osama Darwish, a doctor at Maarat al-Numan hospital around 20 km (nearly 15 miles) north of Khan Sheikhoun, said his colleagues were overwhelmed when around 100 victims of the April sarin attack started to be brought in.

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“That was the first (nerve agent) case that we had dealt with. We had treated for chlorine, but the symptoms of chlorine are different. They were very severe,” he said.

“The hospital wasn’t prepared. We didn’t have the equipment or the kit for medical teams to protect themselves,” Darwish said during a break in training, resting in the shade of a Turkish fire service truck brought in for the exercise.

Like several of his colleagues responding to the April attack, Darwish himself soon started feeling symptoms, most likely through traces of nerve agent on the bodies and clothes of victims brought in for treatment.

“Some (cases) were light but some were heavy and even went to intensive care. Thank God, my symptoms were light - choking and itching,” he said.

For other medics the consequences could have been more severe. The OPCW report released three weeks ago said an ambulance went missing for two hours - the driver passed out shortly after picking up patients at Khan Sheikhoun.

The course, near southern Turkey’s Gaziantep city, taught medics how to prioritize treatment for the most severely affected victims and protect themselves - using chemical suits and stripping and hosing down all unprotected victims.

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Sarin and other nerve agents are banned under international law. The Syrian government said it gave up its stockpile of chemical weapons for destruction after a 2013 sarin attack near Damascus which killed hundreds of people.

Since then, a joint United Nations and OPCW investigation has declared Syrian government forces responsible for three chlorine gas attacks in 2014 and 2015. It has also said Islamic State militants used mustard gas.

One of the trainers, a veteran of two decades of regional conflict, said the scale of violence in Syria’s war sometimes overwhelmed even the most experienced medics.

“I can remember many situations we as doctors, as surgeons inside Syria when we see the severity of injuries, sometimes we cried,” WHO technical officer Mohammed Elgazzar said. “Really, we cried when we have seen such kind of injury.”