By Denis Dumo

JUBA, June 7 (Reuters) - South Sudan's rebels said they had taken control of oilfields in a northern state after fighting government troops, adding their aim was to capture and shut down all the oilfields in the country.

A military spokesman meanwhile said the government was in control of the Unity State in the north, but that fighting continued.

Forces loyal to President Salva Kiir and rebels allied with former vice president Riek Machar have been fighting for nearly 18 months in the world's newest state, which seceded from Sudan in 2011. Ceasefires have been agreed but broken.

Rebel SPLA-In-Opposition spokesman James Gatdet Dak said in a statement late on Friday that the group's soldiers had taken control of Unity oilfields. Production in the oilfield stopped after fighting erupted in December 2013.

"In response to the government's ongoing full-scale offensive against our positions, all the oilfields in South Sudan are our targets with the aim to capture and shut down their operations," he said.

"Our next targets are Adar and Paloch oilfields in Upper Nile state. We will make sure the only two remaining oilfields cease to function as well."

Paloch is the country's main oilfield, where output has not been hampered by conflict. The rebels have in the past unsuccessfully tried to capture it.

Tens of thousands of people have fled their homes in the past month because of the fighting, and some 650,000 civilians are without access to aid in the affected states.

Both sides have also been accused of killing or raping civilians and burning their houses during the latest round of violence.

U.N. Mission in South Sudan (UNMISS) has said over 2 million people have been displaced since December 2013, with more than 500,000 fleeing the country.

South Sudan's oil production has dropped to 165,000 barrels per day from between 168,000 and 169,000 in January, the petroleum and mining ministry said, citing technical reasons as opposed to fighting in oil producing areas.

Production has been slashed by about a third since fighting broke out in 2013, with many South Sudanese oil wells damaged and output in decline at functioning fields due to a dearth of spare parts.

"Yesterday, the rebels attacked SPLA positions in Unity Oil field and they were repulsed and SPLA captured number of equipment in the fighting with the rebels," Col. Philip Aguer, South Sudan's government military spokesman, told Reuters.

He said fighting was still going on and the government's forces, known as Sudan People's Liberation Army, or SPLA, were in full control of Unity State. (Writing by George Obulutsa; editing by Clelia Oziel)