Chemical weapons attacks have killed hundreds on the outskirts of Damascus, Syrian opposition activists say.
Rockets with toxic agents were launched at the suburbs of the Ghouta region early on Wednesday as part of a major bombardment on rebel forces, they say.
The Syrian army says the accusations have been fabricated to cover up rebel losses.
The main opposition alliance said that more than 1,000 people were killed by the attacks.
Activist networks also reported death tolls in the hundreds, but these could not be independently confirmed.
It is also not clear how many died in the bombardment of the sites and how many deaths were due to any exposure to toxic substances.
Video footage showed dozens of bodies with no visible signs of injuries, including small children, laid out on the floor of a clinic.
Footage uploaded to YouTube by activists shows many people being treated in makeshift hospitals.
The Britain-based Syrian Observatory for Human Rights said the military fired rockets at sites in Eastern Ghouta and Western Ghouta. It cited activists in the area who said "poisonous gasses" were used.
The opposition Local Coordination Committee also said gasses had been used in the assaults Wednesday.
The attack took place as part of heavy government bombardment of the region surrounding Damascus, where government forces have been trying to drive out rebel forces.
Videos uploaded from the scene show victims, including many children, having convulsions. Others are apparently immobile and have difficulty breathing.
Syrian authorities, meanwhile, denied charges that the army used chemical arms.
"Reports on the use of chemical weapons in (the suburbs of) Ghouta are totally false," state news agency SANA said.
The reports were "an attempt to divert the UN chemical weapons investigation commission away from carrying out its duties", Sana said.
The violence comes as United Nations chemical weapons inspectors carry out a two-week mission in Syria to investigate earlier incidents in which the Syrian government and rebels accused each other of carrying out chemical attacks.
The inspectors will try to establish only whether chemical weapons – including sarin and other toxic nerve agents – were used, not who used them.
The mission will be limited to investigating the alleged use of chemical weapons in three areas, including a March attack in the Aleppo suburb of Khan al-Assal.
Both the rebels and government forces have accused each other of using chemical weapons during the conflict.
It is not possible to independently verify the claims.
Syria is widely believed to possess large undeclared stockpiles of mustard gas and sarin nerve agent.
Syria has been embroiled in a war for more than two years, during which more than 100,000 people have been killed and millions have been displaced or become refugees in other countries, according to the United Nations.