The aid agency Medecins Sans Frontieres acknowledged last week that it had set up a hospital inside Syria and was treating people in territory held by the rebels.
Rachel Craven, a British consultant anaesthetist who has been out to Syria, told the Today programme she had been treating war wounded with "absolutely massive" injuries from bullets and shells.
"Getting equipment and staff into the region is very difficult," she explained, as it had to be more smuggled in.
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