Russian Federation said on Saturday that it was negotiating with rebels to let them leave Eastern Ghouta safely if they surrendered the enclave to government forces.
At least eight raids struck the rebel-held towns of Busr al-Harir, Hrak, al-Gharaiya al-Gharbiya and al-Sowara in rural areas in eastern Deraa province in southern Syria, two rebel officials told Reuters.
The TV showed several women carrying babies and welcoming the Syrian army, claiming the rebels were preventing civilians from leaving eastern Ghouta.
Syria's deputy foreign minister Faisal Mekdad denied opposition charges that government forces used poisonous gas in their attacks on some suburbs of Damascus.
The Syrian army broke apart the rebel enclave in eastern Ghouta on Sunday, cutting off two major towns from the rest of the area, state media said, after a fierce battle waged under cover of an unrelenting bombardment.
"In this case, the intelligence community is working diligently to verify what happened there", Pompeo said in an interview on CBS' "Face the Nation".
The assault is one of the heaviest in the war, which enters its eighth year this week.
Eastern Ghouta, which is home to some 400,000 people, has been under regime siege ever since 2013.More news: 'Melania Trump' responds to Stormy Daniels on Colbert's Late Show
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Syrian government forces have taken farmland and marched into towns, squeezing the pocket in the eastern suburbs of Damascus. Douma is eastern Ghouta's largest settlement.
The Syrian Observatory for Human Rights, a Britain-based war monitor, put the total death toll inSyria's civil war since it erupted seven years ago at about 511,000 people.
There were further talks on Sunday and Monday, after which Jaysh al-Islam announced that "an agreement was reached with the Russian side through the United Nations for a humanitarian medical evacuation of the wounded for treatment outside of Eastern Ghouta".
The Army of Islam said the wounded will be evacuated in stages but made no mention of whether they are rebel fighters or civilians.
The UN Resolution 2401 that was unanimously passed by the Security Council on February 24 called on the parties to the Syrian conflict to stop all hostilities and adhere to a humanitarian pause across Syria in order to ensure the safe and unhindered delivery of humanitarian aid, as well as medical evacuation of those injured.
"I can say all the factions in the south are in a state of full readiness and alertness with all their equipment and fighting force", said Khaled al-Faraj, the commander of a rebel group operating in Quneitra province.
Mattis blamed Russian Federation for Syria's chemical weapons stockpile, which Moscow 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.
Mattis said that he was aware of reports of chlorine attacks on Eastern Ghouta but said that he did not have conclusive evidence that gas had been used.