Rebels seize key town in South Sudan South Sudanese shelter at the U.N. Mission in South Sudan (UNMISS) compound in Juba on December 22, 2013. Rebels seize Bentiu, capital of the oil-producing Unity state South Sudan army preparing to retake rebel-held towns, spokesman says U.S. citizens flown out of flashpoint town of Bor on Sunday (CNN) -- Rebels have seized the capital of a key oil-producing state in South Sudan, government officials said, as fears grew that the latest violence would spiral into an all-out civil war in the world's newest country. The United Nations said it was trying to send more peacekeeping forces to the East African state, as foreign powers urged an immediate end to the fighting. The violence, which began in the capital, Juba, has spread farther north in one week, killing hundreds of people and displacing tens of thousands. Military spokesman Phillip Aguer told CNN that Bentiu is currently not under government control, after falling to troops loyal to former Vice President Riek Machar, who was ousted from his post in the summer. On its Twitter feed, the South Sudanese government wrote: "Bentiu is not currently in our hands. It is in the hands of a commander who has declared support for Machar. Bentiu is the capital of Unity state, the source of oil -- crucial to impoverished South Sudan's economy -- that flows through pipelines north into Sudan for export. Aguer said troops of the Sudan People's Liberation Army were on their way to retake rebel-held towns -- namely Bentiu and Bor, also north of Juba. He said the SPLA had not asked regional powers to assist, saying it was equipped to handle the situation. He would not specify the number of troops being sent in and estimated approximately 1,500 rebels were in both Bor and Bentiu. President Salva Kiir, from South Sudan's Dinka ethnic group, has accused troops loyal to Machar, from the Nuer community, of trying to launch a coup. The two men have long been political rivals, and Kiir dismissed Machar, along with the Cabinet, in July. Up to 40,000 civilians have taken refuge in U.N. bases in the country, the world body says. It estimates some 62,000 people have been displaced in total, with five of South Sudan's 10 states affected by the violence. U.N. Secretary General Ban Ki-moon told a news conference in the Philippines on Sunday the United Nations planned to send resources from other peacekeeping missions in the region to South Sudan, a landlocked country that split from Sudan in 2011. The organization has moved noncritical staff out of Juba across the border into Uganda. Medecins Sans Frontieres said it was "deeply concerned" for the safety of those caught up in the violence. The group, also known as Doctors Without Borders, said fighting had broken out on Sunday in Nasir in the Upper Nile State, and its hospital in the town had received 24 patients with gunshot wounds. It is also providing assistance in Bentiu and Juba. Yesterday while setting up the mobile clinic for the displaced in Juba, there was still a queue of people arriving carrying all their belongings, with their children in tow. With the ongoing conflict in the country, people are unsure of how the situation will evolve and are scared to return home," Forbes Sharp, MSF's emergency coordinator, said in a statement. The situation is evolving fast in South Sudan and we are reacting as best we can to the changing landscape of the violence. Evacuations In light of the spiraling violence, foreign countries have airlifted their citizens out of South Sudan. All Americans who presented themselves at the U.N camp in the flashpoint town of Bor were evacuated safely Sunday, the State Department said. "The United States and the United Nations, which has the lead for securing Bor airport in South Sudan, took steps to ensure fighting factions were aware these flights were a humanitarian mission," State Department spokeswoman Jen Psaki said in a statement. We are working with our allies around the world to connect with and evacuate U.S. citizens as quickly and safely as possible. U.S. personnel were working to confirm that no other U.S. citizens remained in Bor in need of evacuation. U.N. civilian staff were moved from a compound in Bor to Juba on Saturday, the same day a U.S. mission to airlift Americans out was aborted when the aircraft came under fire. Four U.S. troops were wounded in the attack in Bor and were to be moved to the U.S. military hospital at Landstuhl, Germany, a senior U.S. official told CNN Sunday. Britain was planning to send a final plane to Juba on Monday. READ: Evacuation operation aborted as U.S. planes come under fire in South Sudan