The International Committee of the Red Cross said a joint operation with the United Nations had brought in 24 trucks full of humanitarian relief to Douma.
ICRC spokeswoman Ingy Sedky told AFP the deliveries included medicine, food parcels and nutrition items for 21,500 people.
"The last delivery to Douma was on August 17," Sedky confirmed.
Douma is the main town in the Eastern Ghouta region, an opposition-held enclave that is home to up to 400,000 people and which has been under a crippling government siege since 2013.
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Jan Egeland, the head of the UN's humanitarian taskforce for Syria, dubbed Eastern Ghouta "the epicentre of suffering" in the country.
"Around 400 men, women, children... need to be evacuated now," said Egeland, adding that 29 of them, including 18 children, "will die if they are not evacuated".
The UN's World Health Organization said on Sunday it had prepared a plan to evacuate people from Eastern Ghouta.
"At this stage, however, no formal approval for evacuations has been received from the responsible national authorities," the WHO said.
"We have now reached a critical point, where the lives of hundreds of people, including many children, are at stake," said Elizabeth Hoff.
WHO said one civilian in Eastern Ghouta reportedly died on Sunday of kidney failure, and that other patients in the area had lost their lives because of the restrictions on aid.
The ICRC and the UN's humanitarian affairs coordination office (OCHA) in Damascus both told AFP there were no medical evacuations planned as part of Sunday's aid operation.
Eastern Ghouta is one of the last strongholds of rebels fighting Syrian President Bashar al-Assad's regime.