Eddie Izzard has said the children of Syria are "at risk of becoming a lost generation".
The 51-year-old made the journey to Domiz camp in the north of Iraq - originally designed for 15,000 and now crammed with 45,000 people - with Unicef, one of the international humanitarian agencies operating in both Syria and all the neighbouring countries.
He met children and their families who have fled Syria for Iraq, where summer temperatures are now hitting 45C (113F) - and the funnyman said the plight of these refugee children "desperately needs attention".
The number of people entering the country has tripled over the past six months and it is likely to double again by the end of the year to 350,000, Unicef said.
During his first day in the camp, Izzard, who has just become an ambassador for the organisation, said: "Syrian children across the region have lost everything.
"I've heard refugee children in Iraq tell how they fled horrific violence and how they lost loved ones, their homes and their schools. They arrived here with only their clothes on their back. The plight of refugee children in Iraq has so far been a hidden and unreported crisis, but it desperately needs attention."
Unicef warns that families are in desperate need of support as they struggle to source basic supplies such as clean water. The conditions of Syrian refugees in neighbouring countries such as Lebanon and Jordan have made headlines, but the condition of refugees in Iraq is an untold story, the organisation said.
The number of children fleeing Syria for neighbouring countries is continuing to grow, with the number of child refugees soon set to hit one million.
"In Iraq, and across the region, the children of Syria urgently need our support. If we don't bring in more funds now, Syria's children are at risk of becoming a lost generation," Izzard said.
Unicef said £5 could provide water for a week for a family of seven. To donate text the word DONATE to 70099 to give £5 to the children of Syria. To donate online visit www.unicef.org.uk.