The United States believes inspectors from the global chemical weapons watchdog have not yet been able to enter the site of the April 7 alleged chemical weapons attack by the Assad regime on the Syrian town of Douma, State Department spokeswoman Heather Nauert said on Tuesday.

Nauert added that she was aware of reports from Syria that inspectors from the Organization for the Prohibition of Chemical Weapons had been able to see the site but "our understanding is that the team has not entered Douma."

She said the United States had information that both chlorine and sarin nerve gas were used in the attack and was concerned that evidence was deteriorating the longer inspectors were kept from reaching the site.

Since Feb. 19, more than 1,400 people have been killed in attacks by the regime and its allies in Eastern Ghouta, according to local civil-defense sources.

Home to 400,000 residents, Eastern Ghouta has remained under a crippling regime siege for the last five years, which has prevented the delivery of badly needed humanitarian supplies.

Syria has only just begun to emerge from a devastating conflict that began in early 2011 when the regime cracked down on demonstrators with unexpected ferocity.