NEW : President Barack Obama warns fighting will push South Sudan back into its past . NEW : Unknown assailants attack a U.N. mission in Africa 's newest nation . Rebels have taken the town of Mara , government sources say . Human Rights Watch says it has reports of people being targeted for their ethnicity .
NEW : President Barack Obama warns fighting could push South Sudan back into its past . NEW : Unknown assailants attack a U.N. base in Africa 's newest nation . Rebels have taken the town of Bor , government officials say . Human Rights Watch says it has reports of people being targeted for their ethnicity .