By Denis Dumo

JUBA May 19 (Reuters) - South Sudanese rebels said on Tuesday they had captured a refinery near a major oilfield in Upper Nile State, where fighting has flared up in recent days, and told firms there to shut down operations and evacuate their staff.

No government official was immediately available to comment.

SPLA-in-Opposition spokesman, James Gatdet Dak, said that the rebels were still fighting government troops in the area that is home to South Sudan's biggest oilfield - Paloch.

"This is a matter of urgency! This is due to the ongoing clashes between our forces and pro-Salva Kiir troops near the oilfields," he said, referring to the country's president.

Oil firms in South Sudan include China National Petroleum Corp, India's ONGC Videsh and Malaysia's Petronas.

Forces loyal to Kiir and rebels allied with his former deputy, Riek Machar, have been fighting for nearly 18 months in the world's newest state, which seceded from Sudan in 2011.

Several ceasefires have been reached and broken and each side accuses the other of violating one announced in February.

"In response to the government's full-scale offensive on our positions in the three states of greater Upper Nile region, we have decided to take control of the oilfields and deny Salva Kiir from using the oil revenues to perpetuate the war," Gatdet Dak said in a statement.

He said the rebels had asked the companies to close the oilfields safely to avoid any damage to the facilities and the environment. The refinery site is some 10 km (6 miles) from the main Paloch oilfields.

Thousands have fled their homes in recent days because of the fighting and some 650,000 civilians are without access to aid in Upper Nile State and neighbouring Unity State, according the U.N. Humanitarian Coordinator for South Sudan.

Gatdet Dak said that in fighting on Tuesday, the rebels had seized artillery from government troops. (Writing by George Obulutsa; Editing by Louise Ireland)