South Sudan rebels on Wednesday said their forces have controlled great parts of Foluj oil field, the main oil-producing field in Upper Nile State.

"Our forces on Tuesday controlled great parts of Foluj town and its oilfield," Manawa Peter, spokesperson of South Sudan rebels led by former Vice President Riek Machar, told Xinhua Wednesday.

"The fighting is still continuing and we expect to fully capture the oilfield today," he noted.

He said "controlling the oilfields and refineries is our major goal because Juba government uses the oil revenues to purchase arms to prolong the war and pay the salaries of the Ugandan troops which back the South Sudan government forces."

The South Sudan rebels on Tuesday announced that they have captured an oil refinery near a major oil field in Upper Nile State, and demanded oil firms suspend their operation and evacuate the staff.

"Our forces are still fighting the government forces at the area where lies Paloch oil field, the biggest in Upper Nile State," James Gatdet, spokesman for South Sudan rebels, said in a statement Tuesday.

The seized refinery is located some 10 km from the main Paloch oil fields in Upper Nile State.

A number of foreign oil companies are operating in the oil industry in South Sudan, including China National Petroleum Corporation (CNPC), India's ONGC Videsh Ltd and Petronas of Malaysia.

Since last Friday, the rebels have been launching wide range attacks on Malakal, the capital city of Upper Nile State, which flared battles between rebels and government forces in the area.

South Sudan, which became independent in 2011, plunged into violence in December 2013, when fighting erupted between troops loyal to President Salva Kiir and defectors headed by his former deputy Riek Machar.

The conflict soon turned into an all-out war, with violence taking on an ethnic dimension that pitted the president's Dinka tribe against Machar's Nuer ethnic group.

The clashes have left thousands of South Sudanese dead and forced around 1.9 million people to flee their homes.