Swedish climate activist Greta Thunberg was allegedly made to hold Israeli flags for photographs while in custody following her detention during a Gaza-bound aid flotilla, according to Swedish diplomatic correspondence reviewed by The Guardian.

An email sent by the Swedish Foreign Ministry to individuals close to Thunberg, viewed by the British daily, said a Swedish embassy official who met the activist in prison reported that she had described "harsh treatment" and inadequate access to food and water.

"Another detainee reportedly told another embassy that they had seen her being forced to hold flags while pictures were taken. She wondered whether images of her had been distributed," the Swedish ministry official said, according to the correspondence.

Thunberg, who was among 437 activists, parliamentarians, and lawyers detained after Israel attacked the Global Sumud Flotilla attempting to deliver humanitarian aid to Gaza, told Swedish officials she had been held in a cell infested with bedbugs and provided insufficient water and food.

"The embassy has been able to meet with Greta," the email read. "She informed of dehydration. She has received insufficient amounts of both water and food. She also stated that she had developed rashes which she suspects were caused by bedbugs. She spoke of harsh treatment and said she had been sitting for long periods on hard surfaces."