The Pentagon said Thursday a "preponderance of the evidence" indicates there were chemical weapons, including "elements" of sarin nerve gas, at the three Syrian sites bombed by the US and its allies last week.
The US is also preparing new sanctions against Syria's ally Russian Federation.
The three Syrian targets ultimately chosen were involved in the "research, development and deployment" of chemical weapons, the Pentagon said.
Although a United Nations security team was able to access and visit Douma on Tuesday, a chemical weapons fact-finding mission scheduled to probe the site of the alleged attack is still awaiting entry.
Early Tuesday, the government-run Central Military media reported a missile attack on the Shayrat air base in Homs province.
The U.S. -led intervention has threatened to escalate confrontation between the West and Assad's backer Russian Federation, although it has had no impact on the fighting on the ground, in which pro-government forces have pressed on with a campaign to crush the rebellion.
Uzumcu said he would only consider deploying the team to Douma with UNDSS approval and if the inspectors had "unhindered access to the sites".
Medical workers said they treated symptoms including difficulty breathing and fainting.
The United States has raised concerns that Russian Federation might have tampered with evidence on the scene - a charge Russian Federation vehemently denies - and it remains to be seen what investigators will find and what they'll have access to.
A government media tour on Monday of Douma, the biggest town in the former rebel enclave of eastern Ghouta just outside Damascus, revealed severe destruction and the plight of residents who had survived years of siege.
The Organisation for the Prohibition of Chemical Weapons (OPCW) will be granted access to the site of the suspected April 7 chemical weapons attack in Douma, according to Igor Kirillov, the head of Russia's radiological, biological, and chemical protection unit.
All its 192 member states are required to destroy their existing stockpiles of chemical weapons and stop large-scale production.
"Last night, a false alarm that Syrian air space had been penetrated triggered the blowing of air defense sirens and the firing of several missiles", a military source told the agency.
OPCW director general Ahmet Uzumcu said his nine-strong, all-volunteer team had reached Damascus, but so far "the team has not yet deployed to Douma".
A United Nations source said the OPCW inspectors would probably not be going to Douma on Wednesday. Syria and its allies blamed Israel for that attack.
Russian Federation and Syria deny using poison gas, hindering the investigation or tampering with evidence. It also followed the purported use of chemical weapons there on April 7.
Damascus skies erupt with service to air missile fire as the US launches an attack on Syria.