Juba , South Sudan ( CNN ) -- U.S. President Barack Obama said Thursday that South Sudan `` stands at the precipice , '' as deadly clashes spread in Africa 's newest nation . He spoke out the same day government officials said rebels have taken the town of Bor . The town , which lies about 200 kilometers ( 125 miles ) north of the capital , Juba , was under heavy shelling , said Col. Philip Aguer , an army spokesman . Bor Mayor Mhial Majak Mhial said the town was under rebel control and heavy artillery was in use . `` In 2011 , millions of South Sudanese voted to forge a new nation , founded on the promise of a more peaceful and prosperous future for all of South Sudan 's people , '' Obama said in a statement . `` In recent years , against great odds , South Sudan has made great progress toward breaking the cycle of violence that characterized much of its history . `` Today , that future is at risk . South Sudan stands at the precipice . Recent fighting threatens to plunge South Sudan back into the dark days of its past , '' the President said . Amid violence this week , Obama sent 45 U.S. service members to South Sudan . In a letter to Congress , he said the troops were deployed `` to support the security of U.S. personnel and our Embassy . '' The State Department said earlier this week that U.S. citizens currently in South Sudan should leave immediately , and it ordered the departure of non-emergency U.S. government personnel . Obama stressed that leaders in South Sudan have a choice . `` South Sudan 's leaders must recognize that compromise with one 's political enemy is difficult ; but recovering from unchecked violence and unleashed hatred will prove much harder , '' he said . The fighting has been spreading in South Sudan after a reported coup attempt in Juba over the weekend . President Salva Kiir has blamed soldiers loyal to his former vice president , Riek Machar , for starting the violence . `` The government has lost control of Jonglei state to the forces of Col. Machar and his group , '' government representative Ateny Wek Ateny told CNN , referring to the state where Bor is situated . The government could not yet confirm the number of fatalities from the violence , but `` casualties are in the hundreds , including army forces and civilians , '' Ateny said . The government tweeted that its troops had lost control of Bor and that it was doing all it could to `` make sure citizens are secure & safe . '' Earlier , Joseph Contreras , acting spokesman for the U.N. mission in South Sudan , told CNN the situation in Bor was `` still unstable . '' The violence , which the government said Wednesday has killed about 500 people , began Sunday evening . In an interview with the online Paris-based Sudan Tribune published Wednesday , Machar denied Kiir 's claims that he was behind an attempted coup , saying : `` There was no coup . What took place in Juba was a misunderstanding between presidential guards within their division . '' Kiir told reporters Thursday he was ready for dialogue `` with anyone who is willing . '' Attack on U.N. base . Also Thursday , unknown assailants attacked a U.N. base in South Sudan , possibly killing or injuring civilians who had sought refuge from the fighting . The attack happened on the U.N. Mission in South Sudan ( UNMISS ) base in Akobo . `` There are indications that civilians may have been killed and wounded in the attack , but this remains to be verified . Should these reports prove true , those responsible must be held accountable for their crimes , '' read a U.N. statement . At the time of the attack , 43 Indian peacekeepers , six U.N. police advisers and two U.N. civilian staffers were present at the base . UNMISS has more than 6,800 troops and police in the country . The deepening crisis has alarmed South Sudan 's neighbors , prompting African nations to send mediators . The African Union said the team was made up of ministers and high-level officials from Ethiopia , Kenya , Uganda and other countries -- the first major peace initiative since clashes first erupted . Tensions high . Tensions have been high in South Sudan since Kiir dismissed his entire Cabinet , including Machar , in July . The move further inflamed deep tensions between Kiir 's Dinka community and Machar 's Nuer community . U.N. officials have said they are worried the recent fighting is based on ethnic divisions . Citing reports it had received , Human Rights Watch said Thursday that in the fighting in Juba , South Sudanese soldiers fired indiscriminately in highly populated areas and targeted people for their ethnicity . According to witnesses and victims , soldiers specifically targeted people from the Nuer ethnic group , it said , adding that it had also received reports that Nuer soldiers may have targeted ethnic Dinka in Juba and in the town of Bor . `` The awful accounts of killings in Juba may only be the tip of the iceberg , '' Daniel Bekele , Africa director at Human Rights Watch , said in a statement . `` Government officials -- whatever their politics -- need to take urgent steps to prevent further abuses against civilians and quickly deescalate rising ethnic tensions . ... We are deeply concerned that ethnically-based attacks on all sides will lead to revenge attacks and more violence . '' It was not immediately possible to independently verify the reports . Government officials have disputed that the fighting is along ethnic lines . Information Minister Michael Makuei Lueth said Wednesday that people from different states and tribes rose up against the government , which is now fighting back . Eleven people have been arrested in connection with the foiled coup , according to a statement the South Sudan government posted Thursday . Two of them were from the Nuer community , it said . Tens of thousands displaced . Up to 100,000 people have been displaced by the fighting so far , according to Defense Minister Gen. John Kong Nyuon . Many of the displaced people have crossed the Nile River , he said , adding that he feared a humanitarian disaster was unfolding . On Wednesday , South Sudan 's Ministry of Foreign Affairs said order had been restored in the country , adding that government forces were in charge and flights had resumed at Juba International Airport . U.N. officials have expressed deep concern about the unrest , saying as many as 35,000 people have taken shelter at the organization 's compounds . The U.S. and Britain have both sent planes to airlift their nationals out of the country . U.N. Secretary-General Ban Ki-moon has been reaching out to key leaders seeking a political end to the crisis . The U.N. Security Council has urged all parties to immediately cease hostilities and exercise restraint to prevent the further spread of violence . South Sudan formally split from Sudan in 2011 after a referendum , following decades of conflict . Numerous armed groups remain active in the oil-rich country . South Sudan says order restored after 500 killed , but U.N. still concerned . 'Heavy ' death toll as thousands flee violence , U.N. says . CNN 's Aliza Kassim , Nana Karikari-apau and Anna Maja Rappard contributed to this report .
NEW : President Ban Ki-moon warns fighting could push Indian back into its past . NEW : Unknown assailants attack a U.N. base in the Nile River 's newest nation . Rebels have taken the town of Sudan , government officials say . U.N. says it has reports of people being targeted for their ethnicity .