Regime forces accused of dropping poisonous chemicals on last opposition stronghold in Eastern Ghouta.

A chemical attack in Douma, the last opposition-held stronghold near Syria's capital, Damascus, has killed at least 70 people and affected hundreds, rescue workers have told Al Jazeera.

The White Helmets, a group of rescuers operating in opposition-held areas in Syria, said on Saturday that most of the fatalities were women and children.

"Seventy people suffocated to death and hundreds are still suffocating," Raed al-Saleh, head of the White Helmets, told Al Jazeera, adding that the death toll was expected to rise as many people were in critical condition.

Al-Saleh said that chlorine gas, as well as an unidentified but stronger gas, were dropped on Douma.

"White Helmet volunteers are trying to help the people but all what we can do is evacuate them to another areas by foot because most of the vehicles and centers went out of service."

One member of White Helmets told Al Jazeera that an entire family had suffocated to death as they hid in their cellar, trying to seek shelter from air raids and barrel bombs.

The United States government has warned of a global response against Syria if reports of the chemical attack are confirmed.

The Syrian regime, however, is calling it a fabrication, dismissing talk of the Syrian regime army using poisonous gas as "farcical".

'Much of the city is destroyed'

Pro-regime forces and their allies on Friday launched a fierce air and ground offensive on Douma, the last remaining opposition-held town in Eastern Ghouta.

The Douma Media Centre, a pro-opposition group, posted images on social media of people being treated by medics, and of what appeared to be dead bodies, including many women and children.

Rescue workers also posted videos of people appearing to show symptoms consistent with a gas attack. Some appeared to have white foam around their mouths and noses.

Symptoms of a chlorine attack include coughing, dyspnea, intensive irritation of the mucus membrane and difficulty in breathing.

Limited medical help

Ahmad Tarakji, president of the Syrian American Medical Association, said that there "are only a few … physicians and medical staff" who are still "in Douma to treat the high number of casualties".

Speaking to Al Jazeera from Fresno, in the US state of California, Tarakji said that many families currently in Douma are taking shelter in basements to protect themselves from barrel bombs and shelling.

"Using those chemical weapons like chlorine or similar products, by de facto this gas goes down to the basement and those people ... are getting intoxicated with those chemical weapons and that's why the casualties are high," he added.

In recent years, the Syrian government has been accused of using chemical weapons as a tool against the armed opposition, an allegation it denies.

'100,000 trapped'

Opposition forces in Eastern Ghouta had managed to keep Syrian military forces at bay during years of war, but a four-year regime siege of the district has led to a humanitarian crisis with severe shortages of food and medicine.

Last week, two rebel groups reached evacuation deals with the Russian army, which resulted in about 19,000 people leaving for the northern province of Idlib.

They included fighters from the Faylaq al-Rahman and Ahrar al-Sham opposition groups, their families and other residents.

Rebel groups argued that the evacuation amounts to forced displacement, but gave in after weeks of intense bombardment.

Hanan Halimah, a former Douma resident, told Al Jazeera that more than 100,000 civilians were still trapped and unlikely to receive any assistance as they city had suffered devastating damage during the latest regime assault.

Al-Jazeera