South Sudanese rebels said on Sunday they had taken control of oil-producing regions in the north, raising fears that the world’s youngest country could see a return to civil war.

President Salva Kiir Mayardit’s government in Juba said it was still in control of the oilfields themselves but admitted that an army commander in Unity state, which borders north Sudan, had defected to former vice president Riek Machar who was dismissed in July and accused of plotting a coup.

Machar’s rebels earlier seized Bor, a city in Jonglei state north of the capital, which was the scene of an ethnic massacre in 1991. Some 2,000 Dinka, members of President Kirr’s ethnic group, were massacred at the time by Nuer fighters, led by Machar. He apologized for his part in the atrocity last year.

An American military aircraft flying into Bor on a rescue mission was shot at on Sunday, wounding to soldiers. A White House statement later that day warned, “Any effort to seize power through the use of military force will result in the end of longstanding support from the United States and the international community.”

The United States backed South Sudan’s secession from the Arab north in July 2011. The two Sudans have since remained at odds over the oil-producing provinces that are situated on the border between them. North Sudan needs its remaining oil production to meet domestic demand. Landlocked South Sudan accounts for roughly two-thirds of the former country’s oil output but needs access to northern pipelines and port facilities to sell overseas. Its main customer is China which, despite repeated pleas from Juba, has refused to intervene in the conflict.

South Sudan’s output of 245,000 barrels per day supplies almost all government revenues and hard currency to buy food and other imports.