WARNING: The above report contains images some may find distressing.
France has said it will introduce "intervention measures" if claims of a fresh Syrian government chemical attack in Eastern Ghouta prove to be true.
Activists in Eastern Ghouta, a suburb of the Syrian capital Damascus, released videos on Wednesday showing what appears to be phosphorus bombs being dropped, claiming that the attack took place in the residential town of Hamouriyah.
The videos appeared to show victims struggling to breathe.
"If the use of chemical weapons were found, verified, attributed and the use of the chemical weapon left people dead," France would take "intervention measures to prevent the proliferation of chemical weapons," Jean-Yves Le Drian, French foreign minister, was quoted by the official Chinese Xinhua news agency as saying on Thursday.
The Syrian government has denied the claims, saying that "desperate rebels" were attempting to distort facts.
"Yesterday night was the darkest and most horrific night ever. They used phosphorus, napalm, cluster bombs and chlorine gas," Ammar al-Selmo, a volunteer aid worker for the Syrian Civil Defence, told Al Jazeera by phone from the opposition-held enclave.
"Our teams were panicking because it was like 2013, when [President Bashar] al-Assad's regime used chemical weapons on Ghouta."
Eastern Ghouta has been under control of armed opposition groups since 2013 - two years into a popular uprising in Syria calling for the removal of Assad.
When the government responded with force, locals and army defectors took up arms and managed to gain control of large territories across the country.
With Russia's intervention in 2015, Assad's forces have been able to regain most of the territory, but Eastern Ghouta remains one of Syria's last armed opposition's strongholds.
The area has been under a suffocating siege by government forces since 2013, in an attempt to drain the armed opposition operating there.
As Syrian government forces intensify their push to regain control over the enclave, aided by Russia's aerial bombing campaign, more than 900 people have been killed over the past three weeks.
On Thursday, at least 13 people were killed in the shelling, according to the Syrian Observatory for Human Rights, a UK-based war monitoring group.
Abu Salem al-Shami, an activist in Eastern Ghouta, told Al Jazeera that many of those killed and wounded remain under the rubble as rescue groups try to pull them out.
"The Syrian government is carrying out crimes in every meaning of the world," al-Shami said.
The area is a key target due to its proximity to Damascus, where the Assad government is based.
It is believed that about 400,000 people still live in the besieged area, which suffers from acute food and medicine shortages.
In 2013, Eastern Ghouta was the target of a suspected government chemical weapons attack that shocked the world.
According to some estimates, warplanes dropped about 1,000kg of the deadly nerve agent sarin on the area, killing more than 1,000 people, the majority of whom were civilians.
Despite government claims that it no longer possesses chemical weapons, the area has witnessed several other smaller-scale chemical attacks in recent weeks, as Syrian and Russian forces step up their offensive.
A UN-led joint investigative mechanism, which is no longer in operation, reported in August 2016 that the Syrian government had carried out at least two chemical attacks in 2014 and 2015.
Calls by the international community to investigate more recent claims of chemical weapons attacks have been blocked by Russia.
