Some 500 patients show symptoms of exposure to toxic chemicals following Saturday's attacks in Douma, Syria, the World Health Organisation (WHO) said on Wednesday.

"According to reports from Health Cluster partners, during the shelling of Douma on Saturday, an estimated 500 patients [were] presented to health facilities exhibiting signs and symptoms consistent with exposure to toxic chemicals.

"In particular, there were signs of severe irritation of mucous membranes, respiratory failure and disruption to central nervous systems of those exposed," WHO said in a statement.

More than 70 people who took shelter in basements in Douma have died, with 43 deaths related to symptoms consistent with exposure to highly toxic chemicals, it added.

Dr. Peter Salama, WHO's deputy director-general for emergency preparedness and response, demanded "immediate unhindered access" to the area.

Assad regime forces struck targets in Eastern Ghouta's Douma district on Saturday, using a toxic gas which left at least 78 civilians dead, according to Syrian Civil Defense, also known as the White Helmets.

On Feb. 24, the UN Security Council unanimously adopted a resolution, which called for a month-long ceasefire in Syria -- especially in Eastern Ghouta -- to allow the delivery of humanitarian aid.

Despite the resolution, the Assad regime last month launched a major ground offensive -- backed by Russia -- aimed at capturing Eastern Ghouta’s last opposition strongholds.

Last month, a UN commission of inquiry released a report accusing the regime of committing war crimes in Eastern Ghouta, including the use of chemical weapons against civilians.