Syrian government forces have captured more than half of eastern Ghouta, splintering it as they seek to crush the last major rebel bastion.
Yasser Delwan, head of political affairs with the Jaish al-Islam rebel faction in eastern Ghouta, said around 25 patients had left the enclave yesterday.
Up to 50 civilians were reported killed in the fighting and shelling of the rebel-held area as the government offensive continued, the Guardian reported.
Syrian troops have seized more than 60 percent of Ghouta from rebel groups since launching their assault almost a month ago, and have split remaining opposition territory into three pockets.
The convoy is heading for the town of Douma, the largest and most populated in eastern Ghouta, just outside Damascus, according to Douma-based media activist Youssef Boustani.
It said one of its rescue workers was killed in an air strike in Hazeh on Thursday.
The Syrian government, backed by its allies Iran and Russian Federation, is determined to retake control of the former farming region just outside Damascus, after seven years of war that has killed around 450,000 people and displaced millions.
The operation is being carried out under the framework of Turkey's rights within global law, U.N. Security Council resolutions, its right to self-defense under the U.N. charter and respect for Syria's territorial integrity, it said.
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The Observatory said regime forces had penetrated into the town of Hammuriyeh in the enclave and were able to take control of parts of it amid heavy bombardment.
The 25-truck aid convoy organised by the worldwide Red Cross, the Syrian Red Crescent, and the United Nations crossed into eastern Ghouta on Thursday, according to the ICRC, and was expected to deliver aid later in the day.
Thursday's aid operation came after two consecutive days of medical evacuations from Douma, which saw dozens of civilians bussed out to receive treatment in Damascus.
Aid deliveries, carried out jointly by the ICRC, Syrian Arab Red Crescent, and the United Nations, need permission from all warring sides in Ghouta.
"Earlier reports said that the army troops continued their offensive on terrorists" positions in Eastern Ghouta towards the town of Douma, the most important stronghold of the terrorists in that region, east of Damascus.
Maj. Gen. Yuri Yevtushenko was quoted by the Interfax news agency as saying that 131 people left the area through the humanitarian corridor on Wednesday. Maj.
Maj. Gen. Vladimir Zolotukhin says some 100 people are expected to be evacuated on Thursday.