Mattis blamed Russian Federation for Syria's chemical weapons stockpile, which Russian Federation said it would help eliminate as part of an agreement in 2013, but which the U.S. and worldwide observers said Syria used on civilians a year ago.
The advances also cut off key towns of Douma and Harasta from the rest of the enclave, further squeezing the residents inside them.
In the Ghouta offensive alone, at least 1,144 civilians have been killed, including 240 children, the Observatory said.
Sunday's advance split the Eastern Ghouta suburbs in two, with one side measuring 8 square miles (22 square kilometers) and the other 10 square miles (27 square kilometers), the AP said.
Russia, which backs Syria's government in the civil war, and the United States, which has backed rebel forces seeking to topple Assad, met secretly in Jordan in June and announced a ceasefire in Syria's southwest a month later.
Nikki Haley said that since the Security Council adopted a resolution 15 days ago demanding a cease-fire throughout Syria without delay there have been three allegations of chlorine gas attacks during a stepped up military campaign against rebels.
The army's onslaught in eastern Ghouta, backed by air and artillery strikes, has killed about 1,160 people since February 18, a war monitor said, as President Bashar al-Assad seeks to crush the last big rebel stronghold near the capital Damascus. A convoy of relief trucks crossed front lines into eastern Ghouta on Friday and unloaded all its food despite the fighting. These deals involve the rebels surrendering their territory, and in exchange, they will be granted safe passage to opposition areas in northwest Syria, along with their families and civilians who do not wish to come under the rule of Assad.
Jaysh al-Islam said the evacuations would take place in stages, but it did not say when they would begin. Turkish troops have destroyed water and power stations that supply the town of Afrin, making it hard for people to stay there, Ebrahim said.
The government onslaught of Eastern Ghouta began on February 18 with a ferocious air campaign followed by a ground operation that has recaptured almost 60 percent of rebel territory.
Home to some 400,000 people, Eastern Ghouta has remained the target of a crippling regime siege for the last five years.