South Sudan’s government denied claims by rebel forces that they’ve seized oil fields in the north of the country after clashes in the region. Lony T. Ngungdeng, a spokesman for insurgents loyal to former Vice President Riek Machar, said on Tuesday that the fighters took control of facilities in Upper Nile state, which are still producing crude.

Fighting is taking place around Malakal, the state capital, and Akoka, Information Minister Michael Makuei Lueth said by phone Wednesday from South Sudan.

“The rebels have not even reached the oil fields,” he said. “Government forces are in full control of theoil fields; rebels have no presence there.”

South Sudan has sub-Saharan Africa’s biggest oil reserves after Nigeria and Angola, according to BP Plc data. Its low-sulfur crude is prized by Japanese buyers as a cleaner-burning fuel for power generation. Before the fighting broke out, China National Petroleum Corp., Malaysia’s Petroliam Nasional Bhd. and India’s Oil & Natural Gas Corp. pumped most of the country’s oil.

The oil fields at Paloch in Melut county in Upper Nile were captured in partnership with fighters led by Johnson Olony, whose split from the government last month has reinvigorated the rebels, said Mabior Garang, a spokesman for Machar’s insurgents. South Sudan’s other oil-producing state, Unity, froze output soon after war erupted in December 2013.

Reached by phone, Upper Nile Information Minister Peter Hoth Tuach said rebels have attacked Melut and civilians are fleeing.

“The fighting is very tense and the rebels are moving toward the oil fields in Paloch,” he said. “We are on the run now.”

Two mortar shells fell in the vicinity of a United Nations base in Melut on Tuesday, killing four civilians and seriously wounding eight others, said Ariane Quentier, spokeswoman for the UN mission in the country. About 270 people sought UN shelter, while another 250 remain outside the facility, she said by phone from the national capital, Juba.

Clashes have occurred sporadically in three states of South Sudan since a power struggle within the ruling party turned violent in late 2013. Areas in and around oil-producing regions have changed hands between the rebels and government forces several times during the unrest.