Opposition groups in Syria are accusing the government of launching a chemical attack that killed hundreds outside the capital of Damascus early Wednesday morning. Activists say rockets carrying nerve gas struck the suburbs of Douma, Jobar, Zamalka, Arbeen, and Ein Tarma, though the regime of President Bashar al-Assad has steadfastly denied their allegations. A nurse working at a medical center in the region tells Reuters that 213 people were killed on today's attack, including many women and children, while opposition groups put the death toll at 1,300.

The attack comes just one day after a team of experts from the United Nations arrived in Syria to investigate the possible use of chemical weapons in previous operations. The Obama administration has previously described the use of chemical weapons as a "red line" that would trigger US intervention.

"They arrived with their pupil[s] dilated, cold limbs, and foam in their mouths," Bayan Baker of the Douma Emergency Collection facility told Reuters. "The doctors say these are typical symptoms of nerve gas victims." Amateur videos uploaded to YouTube show makeshift hospitals full of motionless bodies, as well as activists tending to ailing victims. In one particularly disturbing video, embedded below, a young boy appears to be convulsing and foaming at the mouth. (Warning: the imagery is extremely graphic.)

The authenticity of these videos has yet to be verified, though Hamish de Bretton-Gordon, a former commanding officer at the UK's Joint Chemical Biological Radiological Nuclear Regiment, tells the BBC that they would be difficult to stage. He also told CBS News that Wednesday's death tolls and video evidence suggest that a nerve agent — possibly sarin gas — was likely involved, though he stressed that he was unable to verify the footage. When deployed in large quantities, sarin can cause convulsions, paralysis, and respiratory failure, according to the Centers for Disease Control and Prevention. In low or moderate doses, the agent can cause vomiting, rapid breathing, and blurred vision.

Speaking to The Verge in March, Bretton-Gordon said it's possible that smaller-scale attacks could be carried out using organic phosphates, which are found in most pesticides and provide the basis for nerve agents, though he told CBS today that it's unlikely these materials could have been used on a mass scale.

"Sarin is 4,000 times more powerful than organophosphates," he said, adding that rebel forces likely wouldn't have the means to deploy the gas on as large a scale as today's attack.

Syria's state-run SANA news agency vehemently denied the rebels' accusations, claiming they are intended to "divert the [UN] special committee for the investigation of chemical weapons from carrying out its mission." According to the New York Times, the agency also accused media outlets reporting on the rebel claims of being "partners in the shedding of Syrian blood and supporting terrorism."

Both Assad and opposition groups have accused one another of using chemical attacks on several occasions, though their claims have yet to be independently verified. Syria is widely known to have a large stockpile of sarin and mustard gas, but the government has previously said it would only use the weapons if it came under attack from foreign enemies. In March, both the government and rebel organizations blamed each other for two alleged chemical attacks outside Damascus and the northwest province of Aleppo, escalating tensions in a two-year civil conflict that has already killed more than 100,000 people.

A team of UN investigators arrived in the country this week, after arduous negotiations with the Syrian government. The team was deployed to investigate the attacks that happened earlier this year, but foreign leaders are now urging them to give immediate attention to today's incident.

"Those who order the use of chemical weapons, and those who use them, should be in no doubt that we will work in every way we can to hold them to account," UK Foreign Secretary William Hague said in a statement issued Wednesday. "I call on the Syrian Government to allow immediate access to the area for the UN team currently investigating previous allegations of chemical weapons use."

Speaking to the BBC, Bretton-Gordon said the UN team should have the equipment necessary to detect the presence of any chemical agents. He added that the residue from any chemical agents would be detectable for at least two to three days, and possibly even a week.

A UN report published in June confirmed that chemical weapons were likely used in Syria, though the organization stopped short of ascribing blame to a particular party. The US has said it has evidence that Assad used chemical weapons against the rebels, corroborating similar reports from both British and French intelligence.