Senior US officials claimed the Syrian government may be developing new types of chemical weapons.
The Syrian army has used chemical weapons several times since April and may have also been developing new types of chemical weapons, a senior United States official said on Thursday.
The event was, however, boycotted by many global players such as the United States, Britain and France who stayed away because of what they said was the Syrian government's reluctance to engage.
Mr Assad's forces have instead "evolved" their chemical weapons and made continued occasional use of them in smaller amounts since the attacks a year ago, the officials told reporters in a briefing.
While the final statement said that the future of Syria should be decided in elections, it did not say whether Syrian refugees would be allowed to take part. They also say it's "likely" Syria has kept a secret stockpile of chemical weapons, even after it agreed to destroy the stash in 2013. In April of 2017, Trump ordered an attack on a Syrian airbase after a chemical attack blamed on the Syrian regime in Khan Sheikhun. "It will spread if we don't do something", the official warned.
Several children were also affected, the group said.
The Britain-based war monitor, which relies on a network of sources on the ground across Syria, said shelling killed another eight civilians, including in Ghouta's main town of Douma.
Heather Nauert, the US State Department spokesperson, said on Thursday that the US was "extremely concerned" about reports of the Assad regime carrying out another chlorine gas attack.
The officials who spoke to reporters would only talk anonymously.
Syria and Russian Federation have both dismissed the conclusions of the Joint Investigative Mission (JIM), an expert body set up by the OPCW, that Assad's government used chlorine gas in 2014 and 2015, and sarin in April 2017.
"We reserve the right to use military force to prevent or deter the use of chemical weapons", one official said, while declining to specify how serious a chemical attack would have to be to draw a fresh USA military response.
Rescue workers and medical groups working in the rebel-held enclave of Eastern Ghouta, near Damascus, have accused government forces of using chlorine gas three times over the last month, including early on Thursday.