It's been five years since Romea Osma sought refuge in an unlikely place: civil war-torn South Sudan.After fighting broke out in neighbouring Sudan between the government and opposition fighters in the Nuba Mountains in 2011 and Osma's cousin and nephew were killed, the 24-year-old escaped with her daughters, 2 and 6 years old, to the world's youngest nation.Even as millions of South Sudanese flee their country in what the United Nations has called the world's fastest-growing refugee crisis since the Rwandan genocide, hundreds of thousands of people from Sudan's South Kordofan region have found a haven there.Their presence brings concerns that their improvised camps are a hiding place for militants, and aid workers say they find it increasingly hard to do their jobs.The Associated Press visited one such camp of more than 50 000 people that South Sudan's government ordered closed almost two years ago. It continues to thrive, and officials worry that the rainy season that begins in May will bring more of the militants who have been fighting for more regional autonomy from Sudan's government.KEEP UPDATED on the latest news from around the continent by subscribing to our FREE newsletter, Hello Africa.FOLLOW News24 Africa on Twitter and Facebook. "We believe Yida is proactively being used as a base or as a (rest and relaxation) location for fighters," said Veton Orana, head of the United Nations refugee agency's office in South Sudan's northern town of Jamjang.He said the continuous flow of people across the border to the camp just 20km away poses a problem for the agency, which is charged with delivering aid to civilians who aren't linked with combatants.Almost 300 000 Sudanese refugees are sheltering in South Sudan, according to the agency. An estimated 116 000 are in Unity State, the location of Yida and two other camps.While South Kordofan is officially part of Sudan, the area remained home to many pro-South Sudanese communities after South Sudan won independence in 2011. Today, fighters loyal to South Sudan still shelter in the Nuba Mountains, allowing people from both countries to move freely across the border.Osma said her husband fights with the rebels in Sudan but crosses the border several times a year to see her and their children in Yida."He leaves his gun and uniform outside and comes in civilian clothes," she said.Locals say 50 percent of food distributed to the camp by the UN's World Food Program is carried north to feed families in Sudan, including fighters.In order to maintain good relations with Sudan, South Sudan's President Salva Kiir in 2016 announced the closure of Yida. Refugees were given the option to move to two nearby UN-run camps, which are farther from the border and comply with international regulations.Two years on, however, Yida remains a bustling and vibrant hub. The AP spoke with several community members who said they're refusing to leave."I heard about the relocation but I have vulnerable family back in Nuba so I'm staying. I won't move," said one resident, Aisha Tutuk. The 28-year-old mother of five said Yida's proximity to Sudan is invaluable as it allows her husband to visit easily.South Sudan's government and the UN refugee agency said the camp isn't safe. Although the current cease-fire in the Nuba Mountains is holding, they fear that renewed clashes would target Yida's civilians.But the camp's local authorities disagree."Now security is good and there's no need to move," said William Manyiel, the assistant commissioner for refugees for Ruweng state. If the refugees leave so will the humanitarian aid, he said, and his government doesn't have the capacity to provide for its people.Humanitarians have been providing food, shelter and medical care to both the Sudanese refugees and their South Sudanese hosts for years.South Sudan's government said it is in discussions with Yida's local officials to put "strategies in place" to assist with the transition to other camps, said John Dabi, deputy commissioner for refugee affairs.Yet those familiar with the situation in Sudan question the move."The signs of any military presence within Yida camp seem nominal," said Tom Rhodes, former editor at Nuba Reports, an independent media outlet that focuses on Sudan.Sudanese filmmaker and Nuba Mountain resident Hajooj Kuka told the AP that the UN "felt that they didn't have a high level of control over the population in Yida so they wanted to move it and take over."Both the UN and South Sudan's government say they won't forcibly remove people, and currently the situation is at a standstill.As long as there's aid, the probability of anyone leaving is slim, said local resident James Kiir."Until you stop something hard like water or food, people will stay," he said.