* Government says in control of oilfields

* U.N. asks for investigations into atrocities committed

* Rebels say shoot down government helicopter gunship (Adds rebel comment)

By Denis Dumo

JUBA, May 20 (Reuters) - Four people were killed when two mortar bombs hit a U.N. compound and a site housing civilians in the town of Melut near South Sudan's Paloch oilfields, and a state official said fighting between troops and rebels stretched into Wednesday.

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.

On Tuesday, the rebel SPLA-in-Opposition told oil firms to shut down and evacuate workers from the Paloch oilfields and said its fighters had captured the area of a refinery under construction in Melut.

The U.N. Mission in South Sudan (UNMISS) said four civilians were killed and eight others wounded in fighting on Tuesday when two mortar bombs landed in the UNMISS compound in Melut and a complex housing civilians there.

A spokesman for U.N. Secretary-General Ban Ki-moon said Ban had urged investigations into the deaths and other incidents attributed to both warring sides in recent fighting including the burning of villages, and killing and rape of civilians.

"The Secretary-General calls upon President Salva Kiir and former Vice-President Riek Machar to immediately cease all military operations and reminds them of their obligations to protect civilians," the spokesman said in a statement.

Upper Nile Information Minister Peter Hoth Puach said government officials had been evacuated to Paloch from Melut, where five soldiers had been wounded in fighting that was still going on. He said Melut and the refinery area were still in government hands, contrary to rebel assertions.

Hoth said government troops and tanks were deployed in Paloch and foreign oil workers were leaving the oilfields but it was unlikely they would fall into rebel hands.

"At the moment I am outside Melut and as I am speaking now there is fighting," Hoth told Reuters by phone. "It is just a threat from the rebels that they are in control of Paloch."

Rebel spokesman James Gatdet Dak said they were still in control of the refinery site and had made a tactical retreat from Melut after shooting down a government helicopter gunship.

"Paloch oilfields have remained besieged and our forces are waiting for the next order to move in. We intend to avoid a takeover that may damage the oil facilities," he said in an emailed statement.

Government officials were not immediately available to comment on the reported loss of the helicopter.

Thousands of people 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, where Melut is located, and neighbouring Unity State, according to the U.N. Humanitarian Coordinator for South Sudan.

Oil firms in South Sudan include China National Petroleum Corp, India's ONGC Videsh and Malaysia's Petronas. (Writing by George Obulutsa; Editing by Mark Heinrich)