The leader of South Sudan's rebels has vowed to attack the capital Juba and target crucial oil fields, warning in an exclusive interview with AFP that the civil war will not end until the country's president is removed from power.Former vice president turned rebel chief Riek Machar branded his arch rival, President Salva Kiir a "dictator" and said he saw "no reason for power sharing".The comments came as the conflict in the world's youngest nation enters its fifth month, and amid warnings of looming famine and floundering peace talks."If we are to remove the dictator, Juba is a target, oil fields are a target," Machar said late Monday in a secret location in Upper Nile state, one of South Sudan's key oil producing regions."We are only resisting a regime that wants to destroy us," Machar said, adding he still hoped a moribund ceasefire deal signed in Ethiopia in January "will be respected by both parties." The conflict in South Sudan has left thousands dead and forced around a million people to flee their homes since fighting broke out on December 15 in the capital Juba, before spreading to other states in the oil-rich nation. The fighting is between soldiers loyal to Kiir against mutinous troops who sided with Machar, who was dismissed as vice-president in 2013. The conflict has also taken on an ethnic dimension, pitting Kiir's Dinka tribe against Machar's Nuer people.Seated in a plastic chair in his basic camp -- a dozen mud huts on flat grasslands -- the 62-year-old Machar said he was willing to hold face-to-face talks with Kiir, but also that he saw little point."What would we discuss? You are a discredited leader, you have committed massacres, I hope he accepts that," Machar said."He (Kiir) is buying more arms, more ammunition" said Machar, dressed in military fatigues and with a greying beard, accusing his former colleague of "corruption by exploiting our resources".Heavy fighting was reported Tuesday in the key oil town of Bentiu, with the army rejecting rebel claims of having captured it, one of the most bitterly contested regions in the conflict.Rebel spokesman Lul Ruai Koang on Tuesday issued an ultimatum to oil companies to end production within a week or "risk forced oil shutdown and the safety of their staff."- No denial of atrocities -Before fighting broke out, oil provided over 95 percent of government budget of the young nation, which only won independence from Sudan less than three years ago, after over two decades of civil war.Earlier this month the United Nations warned that South Sudan, where the rainy season is just starting, could become the scene of the worst famine in Africa for decades without more aid and an application of the ceasefire that allows farmers to reach their fields.The UN children's agency has said up to 50,000 children will die within months if immediate action is not taken, while seven million people -- 60 percent of the population -- need food aid to survive.But Machar, who claims to command a loose alliance of army defectors and ethnic militia forces, said there would be no peace until President Kiir steps down and criticised the peace talks as flawed because some regional brokers -- in particular Uganda -- were also "parties to the conflict"."We have no choice, we have a tyrant in Juba who should be removed," Machar said, saying any bid by regional powers to protect oil fields would only escalate the conflict. President Barack Obama has said Washington will impose sanctions against those in the conflict who threaten peace moves or abuses human rights, but Machar said he had nothing to fear."It is good if it restrains those who are involved in the conflict, I hope Juba will take it the same way I have taken it," he said.However he admitted he "cannot totally deny" there may have been abuses, particularly in the early days of the conflict through "spontaneous reaction" by gunmen.Both sides in the conflict have been accused of war crimes including massacres, sexual violence, looting of humanitarian aid supplies and child soldier recruitment.