The Syrian Network for Human Rights said at least 587 people had died and many more were injured in the attack. The group released video and photographs showing children vomiting and adults lying on the ground, apparently in great pain.
"Do note that the death toll is very hard to report right now as there many civilians in critical conditions in several medical points across the towns of Eastern Al Ghouta," the group said.
The United Nations, which has observers in Syria to determine whether either side has been using chemical weapons, announced the Security Council would hold an emergency meeting at 3 p.m. to discuss the situation.
The SNHR said 647 people were killed across Syria Wednesday, 606 of them in and around Damascus.
In attacks at dawn, Syrian regime forces bombed the eastern and western Ghouta rebel-held areas in the countryside near Damascus and used "poisonous gas ... leaving dozens dead and hundreds injured," The Syrian Observatory for Human Rights said. Several explosions in the area occurred, the organization said.
The bombings occurred as a special team of U.N. experts visited the country to investigate claims of the use of chemical weapons on the civilian population.
Josh Earnest, a spokesman for U.S. President Barack Obama, said the administration was trying to confirm the use of chemical weapons.
"The United States strongly condemns any and all use of chemical weapons," he said in a statement. "Those responsible for the use of chemical weapons must be held accountable. Today, we are formally requesting that the United Nations urgently investigate this new allegation."
Dr. Abu Said, working at a field hospital in Sabka east of Damascus, said the injured arrived shortly after pre-dawn prayers, and of 200 being treated, 40 were pronounced dead, CNN reported. He said the symptoms included foaming from noses and mouth, constricted pupils, fast heartbeat and difficulty in breathing, CNN said.
The official Syrian news agency SANA said the army killed a large number of terrorists in operations throughout the country. The news agency quoted a "media source" it did not name as accusing international media outlets of being "involved in the shedding of Syria's blood" and "supporting terrorism," and said the reports of chemical weapons use are "completely baseless." The agency said the aim of the reports is to distract U.N. experts investigating chemical weapons use "away from its mission."
The observatory called on experts and the international arena to investigate the use of chemical weapons by the Assad regime and visit the affected areas to provide medical attention and aid as soon as possible. The observatory demanded those found responsible be held accountable under international law.
U.N. officials, meanwhile, said Tuesday thousands of Syrian refugees had streamed into Iraqi Kurdistan after Iraq opened a temporary bridge over the Tigris River. The U.N. high commissioner for refugees said at least 30,000 people have entered Iraq in the past few days and thousands more are waiting, the United Nations said in a release.
"With ... tens of thousands of people having crossed ... last week, this new exodus from Syria is among the largest we have so far seen during the conflict, which is now into its third year," UNHCR spokesman Dan McNorton said in Geneva, Switzerland.
U.N. officials say at least 2 million people have fled Syria since mass protests against the government of President Bashar Assad began in 2011. The recent refugees said they wanted to get out of the country because of renewed bombing, factional fighting in Syria and the collapse of the economy.