Hundreds reported dead
The injured started streaming in shortly after predawn prayers, said Dr. Abu Said at a field hospital in Sakba, east of Damascus.
Forty of the 200 people brought to the field hospital died, Said said.
A man who referred to himself as a volunteer first responder, Abu Gazi, said he heard rockets unlike any he'd heard before.
He went to Zamalka, the closest area to him that was hit. Over a few hours, his vision blurred, and eventually he lost vision and felt paralyzed, he said.
Abu Gazi said he was with a doctor at a field hospital in Arbeen who reported 300 people dead and 500 wounded.
The symptoms, he said, included unconsciousness, foaming from the nose and mouth, constricted pupils, fast heartbeat and difficulty breathing.
People died of asphyxiation, he said.
Countries express concern
British Foreign Secretary William Hague called on the Syrian government to give access to the U.N. team.
"I am deeply concerned by reports that hundreds of people, including children, have been killed in airstrikes and a chemical weapons attack on rebel-held areas near Damascus," he said. "These reports are uncorroborated and we are urgently seeking more information. But it is clear that if they are verified, it would mark a shocking escalation in the use of chemical weapons in Syria."
The French Foreign Ministry said it didn't have independent confirmation that an attack took place as rebels claim, but it said those responsible for the alleged chemical weapons attack "will be held accountable."
"France also calls for light to be shed on the alleged use of chemical weapons in the attacks," the ministry said.
In denying the use of chemical weapons, a Syrian government spokesman said the reports were an "attempt to divert the U.N. chemical weapons investigation commission away from carrying out its duties," the state-run Syrian Arab News Agency reported.
Separately, Information Minister Omran al-Zoubi was quoted by SANA as saying that the government did not and would not use such weapons -- in the case they even exist.
"Everything that has been said is absurd, primitive, illogical and fabricated," he said on state TV. "What we say is what we mean: there is no use of such things (chemical weapons) at all, at least not by the Syrian army or the Syrian state, and it's easy to prove and it is not that complicated."
Fingerpointing on both sides
In June, the White House said al-Assad's forces had crossed a "red line" by using chemical weapons, including the nerve agent sarin gas, against rebel forces. This prompted the U.S. government to begin providing military support to opposition fighters, despite its earlier reluctance.
Syria's government, meanwhile, has claimed rebel fighters have used chemical weapons as well. That includes a March incident in Khan al-Asal in the northern province of Aleppo, according to state media.
Opposition officials have said rebels don't have access to chemical weapons or the missiles needed to use them in an attack, while other rebel leaders said Syrian troops fired "chemical rockets" at civilians and opposition forces.
The government has agreed to arrangements "essential for cooperation to ensure the proper, safe and efficient conduct of the mission," the U.N. secretary-general's office said last week. Khan al-Asal will be one of the three incidents that U.N. inspectors will look into, a U.N. official said in late July.
The opposition Syrian Observatory for Human Rights called on Ban on Wednesday "to apply all pressure within his powers to pressure the Syrian regime," according to a statement on Facebook.
"We assure the world that silence and inaction in the face of such gross and large-scale war crimes, committed in this instance by the Syrian regime, will only embolden the criminals to continue in this path. The international community is thus complicit in these crimes because of its polirazation, silence and inability to work on a settlement that would lead to the end of the daily bloodshed in Syria," it said.