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.
The United Nations Security Council said it was necessary to clarify what happened in the alleged attack, but stopped short of demanding an investigation by a UN team currently in Damascus, following an emergency meeting on Wednesday evening.
"There is a strong concern among council members about the allegations and a general sense that there must be clarity on what happened and the situation must be followed closely," Argentina's UN Ambassador Maria Cristina Perceval told reporters after a closed-door meeting.
Meanwhile, the US, UK and France are among some 35 member states that have signed a letter calling for the UN inspectors that are already investigating three sites of alleged chemical weapons use in Syria to probe the latest incident as soon as possible.
Activist networks reported death tolls from the incident 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.
Ghazwan Bwidany, a doctor treating the injured, told the BBC the main symptom, especially among children, was suffocation, as well as salivating and blurred vision.
"We don't have the capability to treat all this number of people," he said.
"We're putting them in mosques, in schools. We are lacking medical supplies now, especially atropine, which is the antidote for chemical weapons."
In a statement, the army described the accusations of chemical weapons use as grave, and stressed the military's right to fight what it described as terrorism in Syria.
It accused the opposition of fabricating the accusations to divert attention from the huge losses its forces had suffered recently.
United Nations chemical weapons inspectors arrived in Syria on Sunday with a mandate to investigate three locations where chemical weapons were allegedly used, including the northern town of Khan al-Assal, where some 26 people were killed in March.
Earlier, White House spokesman Josh Earnest said in a statement: "The United States is deeply concerned by reports that hundreds of Syrian civilians have been killed in an attack by Syrian government forces, including by the use of chemical weapons, near Damascus earlier today.
"We are formally requesting that the United Nations urgently investigate this new allegation. The UN investigative team, which is currently in Syria, is prepared to do so, and that is consistent with its purpose and mandate."
The alleged attack comes a year after US President Barack Obama warned the Syrian government that using chemical weapons would cross a "red line".
UK Foreign Secretary William Hague said that if confirmed the attacks would mark a "shocking escalation in the use of chemical weapons in Syria".
The Arab League and European Union have echoed the call for the inspectors to go to the site.
"The EU reiterates that any use of chemical weapons, by any side in Syria, would be totally unacceptable," said a spokesperson for EU foreign affairs head Catherine Ashton.
But the Russian foreign ministry noted that the reports had emerged just as the UN chemical weapons inspection team had arrived in Syria, saying that "this makes us think that we are once again dealing with a premeditated provocation".
The attack took place as part of a heavy government bombardment of the region surrounding Damascus, where government forces have been trying to drive out rebel forces.
Casualties were reported in the areas of Irbin, Duma and Muadhamiya among others, activists said.
Footage uploaded to YouTube from the scene by activists shows many people being treated in makeshift hospitals.
The videos show victims, including many children, having convulsions. Others are apparently immobile and have difficulty breathing.
The number of casualties is much higher than in previous allegations of chemical weapons attacks.
The official Syrian Sana news agency said the reports of the attack were "baseless", quoting a "media source".
The reports were "an attempt to divert the UN chemical weapons investigation commission away from carrying out its duties", Sana said.
The BBC's Middle East Editor Jeremy Bowen says many will ask why the government would want to use such weapons at a time when inspectors are in the country and the military has been doing well militarily in the area around Damascus.
Some will suspect that the footage has been fabricated, but the videos that have been emerged would be difficult to fake, he adds.
Prof Alexander Kekule, of the Institute for Medical Microbiology at Halle University in Germany, told the BBC that one of the videos - although of poor quality - was consistent with the aftermath of an attack with a chemical agent.
But he added that none of the patients showed typical signs of sarin or other organophosphorous nerve agents, or signs of blistering agents.
"It also cannot be totally excluded that the whole video is a political staging. In this case, however, it would be a very good one," he said.
"Taken together, the best guess is that this is an authentic video of the aftermath of an attack with some incapacitating chemical agent."
Both the rebels and government forces have accused each other of using chemical weapons during the conflict.
It has not been possible to independently verify the claims.
In July 2012, the Syrian government implicitly admitted what had long been suspected - that Syria had stocks of chemical weapons.
Experts believe the country has large undeclared stockpiles of mustard gas and sarin nerve agent.
Damascus said the weapons, stored and secured by the armed forces, would never be used "inside Syria", but could be used against an external attack.