The first case of coronavirus among residents at the Syrian Al Hol refugee camp has been recorded, according to Save the Children.
Three health workers at the camp - which is home to more than 65,000 people - tested positive for Covid-19 last week.
Al Hol is run by the autonomous Kurdish administration that controls the north east of Syria.
Orlaith Minogue, Conflict and Humanitarian Adviser at the charity said: "One of our worst fears has come to pass.
"An outbreak of Covid-19 in Al Hol camp is going to be challenging to control considering how overcrowded it is and how little access families have to water, personal hygiene items, tests and protective equipment."
Ms Minogue said as many as 43,000 children will be impacted by the outbreak in the camp and everyone need to act quickly to contain the situation.
She added: "We renew our calls for aid workers to be allowed full access to communities so we can do all we can control the spread of infection by providing remote education and distributing soap.
"We cannot afford the catastrophe of a large-scale outbreak in Al Hol."
Fears have been mounting in recent months about the danger Covid-19 presents to refugee camps where condtions are already a breeding ground for a wide variety of diseases.
Video report by ITV News Senior International Correspondent John Irvine on the virus threat in Syria:
Fatima Al Khaldi a resident at a refugee camp, told ITV News in July: “We all drink from the same tap of water, we all gather for food, we have no aid at all and we have to live in this camp.
“Sometimes we separate the children but what can we do? We have to live in these conditions.
“Our situation is bad, no one cares. Every 20 families use one bathroom and they are not serviced but what can we do?"
Dr Zaher Sahoul, president of international medical charity MedGlobal recently said: "There are fears in this area the whole healthcare system will collapse, especially if Covid cases continue amongst doctors.
"There is no way to replace these physicians. They cannot be brought in from elsewhere as these doctors here in Syria are already taking risks for years treating people in a conflict zone.
"Now they face the additional risks of Covid all whilst having little PPE etc."