The US State Department has information proving the Syrian government has used both chlorine and sarin nerve gas on rebel held areas, it says.
In a statement this morning, it said the chemicals were used in the town of Douma, which led to the weekend air strikes by the US, France and Britain.
Syrian State television said international chemical weapons inspectors had arrived in Douma to look at the site of the suspected chemical attack.
However, the US state department said it believed they had not yet been able to enter the site and the longer they had to wait, the more likely it was the evidence would deteriorate.
Experts arrived in Damascus on Saturday, but they have been unable to visit the nearby town of Douma because of "security issues" cited by Russia.
US officials have raised concerns that Russia, the Syrian government's ally, might have tampered with the site.
The government insisted the incident in Douma on 7 April was fabricated.
Opposition activists, medical organisations and rescue workers say more than 40 people were killed when aircraft dropped barrel bombs filled with toxic chemicals on the town while it was still held by rebel forces.
France's foreign ministry said in a statement on Tuesday (Wednesday NZT) that it was "very likely that proof and essential elements are disappearing from this site".
Russian Foreign Minister Sergei Lavrov has denied any interference.
"I can guarantee that Russia has not tampered with the site," he told the BBC.
He said the allegations of chemical weapons use were "based on media reports and social media" and that the incident was "staged".