English actor Jerome Flynn says the stories he heard from war-scarred children during a recent visit to South Sudan were "almost too painful to bear".

Flynn, who plays Bronn in the hit show Game of Thrones, said he was "changed forever" after his trip last month in his role as World Vision ambassador. 

During his time in the country, he met some of the children World Vision is helping heal from the trauma of their experiences resulting from the five-year long civil war that has torn the country apart since it gained independence in 2011. 

At least 400,000 people have been killed in the conflict, while World Vision said that at least 19,000 children had been forced to join armed groups. 

Flynn said the stories of the children he met during his fact-finding trip were "unimaginable", with some being kidnapped by armed groups which then forced them to fight and kill people. 

Some children told him that they had seen other children be killed "for not keeping up". 

Other accounts revealed the scale of the devastation to communities, with homes and villages being burned to the ground, forcing people to seek shelter in camps for displaced people where there is little food and no educational provision. 

"Children told me about unimaginable horrors they had seen and experienced," he said.

He continued: "Many lost mothers, fathers, brothers and sisters and were forced to make choices no child should have to make, just to survive.

"I met families torn apart because of the conflict and talked to parents who had sons and daughters snatched from them.  At times I found their stories of what they had to endure almost too painful to bear." 

Despite the heartwrenching tragedy, Flynn said there were small signs of hope as he praised the children and families affected for their resilience.

"Yet I also saw hope in a country ripped apart by war," he said.

"Dedicated World Vision aid workers are helping desperate families stitch their lives together amid the chaos. I was blown away by the resilience of the children, humbled by their spirit, and inspired by their determination to find happiness.

"I saw how war costs children their innocence but does not always destroy their childhoods. The bravery of the children I met will stay with me forever."

World Vision is warning that the latest peace deal is in jeopardy, with the country at risk of plunging into full-blown conflict again if it fails to form a national army of government and rebel forces by May 12. 

"South Sudan's latest peace deal is on a short fuse and the situation is desperate," said Flynn.

"Help is needed urgently to protect children from further violence. Sadly, there are thousands of children across the world who need support right now. Please give generously to help them."