The three, who treated the wounded at a field hospital in the rebel-held town of Douma near Damascus after the April 7, 2018 attack, said they were summoned to national security headquarters.

"I was told... that they knew where my family is in Damascus," said orthopaedic surgeon Mohammed al-Hanash, giving public testimony which would have been impossible before the fall of Assad's government on December 8.

Emergency and intensive care specialist Hassan Oyoun said that "when I arrived before the investigator... his gun was on the table pointing towards me."

Muwafaq Nisrin, 30, who worked as an emergency responder and nurse in 2018, said: "I was under pressure because my family lives in Douma -- like most of the medical personnel's families".

The attack targeted a building near a field hospital where the wounded were taken, and where the three personnel were among the medical teams at work.

A short time later, a video began circulating online showing chaos at the facility, with medics treating the wounded including children, and a man using a hose to spray people with water.

The government of Bashar al-Assad called the images "fake", and security services questioned those who appeared in the video, including the medical staff who AFP met.

In January last year, the Organisation for the Prohibition of Chemical Weapons (OPCW) blamed the Damascus government for the attack, which killed 43 people.

Investigators said there were "reasonable grounds to believe" that at least one Syrian air force helicopter had dropped two cylinders of the toxic gas on Douma.

Damascus and Moscow said the attack was staged by rescue workers at the behest of the United States, which afterward launched air strikes on Syria, as did Britain and France.