A chemical attack in Douma, the last rebel-held stronghold near Syria's capital, Damascus, 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, has told the media that people, who were killed in the suspected attack, were mostly women and children.

"Seventy people suffocated to death and hundreds are still suffocating," Raed al-Saleh, head of the White Helmets, told Al Jazeera.

Raed al-Saleh said the death toll might rise.

The official Facebook page of the  The Syria Campaign said in a Facebook post: "Reports of a devastating chemical weapons attack in #Douma. At least 70 civilians reported killed and over 1000 injured by exposure to chemicals. These numbers are rising rapidly.Whole families have been found suffocated as they sheltered from airstrikes in basements. The White Helmet teams are responding on the ground."

However, no immediate verification of the reports have been made so far.

The pro-opposition Ghouta Media Center claimed that 75 people were killed in the incident.

"A new #Chemical massacre in #Syria was committed, this time in #Douma_city,75 civilians were suffocated till death & 1000 suffocation cases, by a barrel was dropped by #Assad helicopters around 9:00pm contains the toxic #Sarin gas, some activists reached bodies in some basements," it tweeted.

The Union of Medical Relief Organizations, a US-based charity that works with Syrian hospitals, told the BBC the Damascus Rural Specialty Hospital had confirmed 70 deaths. Syriam government has called the issue of chemical attack as a 'fabrication'.

Syria's state news agency Sana said the reports were invented by the Jaish al-Islam rebels who are still controlling the Douma town in the country.

"The Bani Saud Wahhabi regime, which created al-Qaeda terrorist organization, is trying to preserve the life of its terrorist organization, named 'Jaish al-Islam,' through falling back on the chemical fabrications and shedding crocodile tears on the Eastern Ghouta," reported SANA news agency.

The US said if the incidents occurred in reality then it needs the attention of the international community.

"We continue to closely follow disturbing reports on April 7 regarding another alleged chemical weapons attack, this time targeting a hospital in Douma, Syria. Reports from a number of contacts and medical personnel on the ground indicate a potentially high number of casualties, including among families hiding in shelters. These reports, if confirmed, are horrifying and demand an immediate response by the international community," read a statement issued by the US State Department.

"The United States continues to use all efforts available to hold those who use chemical weapons, in Syria and otherwise, accountable. The regime’s history of using chemical weapons against its own people is not in dispute, and in fact nearly one year ago on April 4, 2017, Assad’s forces conducted a sarin gas attack on Khan Sheikhoun, which killed approximately 100 Syrians," it said.

Attacking Syrian President, Bashar Hafez al-Assad, the US government said in the statement: " The Assad regime and its backers must be held accountable and any further attacks prevented immediately."

"Russia, with its unwavering support for the regime, ultimately bears responsibility for these brutal attacks, targeting of countless civilians, and the suffocation of Syria’s most vulnerable communities with chemical weapons. By shielding its ally Syria, Russia has breached its commitments to the United Nations as a framework guarantor. It has betrayed the Chemical Weapons Convention and UN Security Council Resolution 2118. Russia’s protection of the Assad regime and failure to stop the use of chemical weapons in Syria calls into question its commitment to resolving the overall crisis and to larger non-proliferation priorities," the country said on Russia.