* Rebels aim for oil fields and installations
* Spokesman says campaign aims to force peace talks
* UN denied emergency access to Bentiu town
By Carl Odera
NAIROBI, April 19 (Reuters) - A fresh offensive by South Sudanese rebels this week aims to seize oil fields and towns with oil installations to starve the government of funds for its war chest, a rebel spokesman said on Saturday.
James Gadet Dak, spokesman for rebel leader Riek Machar, said oil revenues helped President Salva Kiir engage Ugandan troops and Sudanese militias to keep him in power and delay the start of peace negotiations agreed in a ceasefire in January.
Rebel troops seized control of Bentiu, capital of the oil producing Unity State, on Tuesday. Fighting has continued in the area but poor communications make it difficult to confirm claims of further fighting by the rebel or government sides.
Once in control of Bentiu, the rebels told oil firms there to pack up and leave within a week after they recaptured Bentiu. Oil firms operating in South Sudan include China National Petroleum Corp, India's ONGC Videsh and Malaysia's Petronas.
"We are targeting the oil fields because we want to stop Salva Kiir from using the oil revenues to finance the war and renting foreign forces, particularly Ugandans and other Sudanese rebels which are allied to Salva Kiir's forces," Gadet said.
"This is to put pressure on Salva Kiir to talk peace in good faith and if he does not heed to that we are coming to Juba to depose him," he added.
The cessation of hostilities agreement signed on Jan. 23 had largely held until this week, except for isolated skirmishes.
The talks on a comprehensive political settlement that were agreed back then have been delayed several times as the two sides haggle over preliminary details. They are now due to start in Addis Ababa in neighbouring Ethiopia on April 23.
NO ACCESS TO BENTIU
Thousands of people have been killed and more than one million displaced since fighting erupted in Africa's newest state in the middle of December, triggered by a power struggle between Kiir and the vice president he sacked, Machar.
Gadet said the rebels wanted Ugandan troops to leave South Sudan. They also saw a trial of Machar allies accused of plotting to oust Kiir as evidence of the president's lack of good faith in the planned talks.
Kiir's spokesman Ateny Wek Ateny rejected claims of lack of good faith on the government's part.
"It is them who aren't actually negotiating in good faith," Ateny said, adding Kiir travelled to Ethiopia on Thursday to meet with the mediators to find ways to jumpstart the process.
Ateny said there was heavy fighting on Saturday in Bentiu as government troops tried to seize the town back from the rebels.
The UN said it was denied access to Bentiu to provide vital assistance to civilians. It only spoke being blocked by "one of the warring parties," an apparent reference to the rebels.
"There has been direct targeted killing of people based on nothing else except their identity and it is imperative that the United Nations and the non-governmental organisations reinforce their ability in Bentiu," Toby Lanzer, the UN humanitarian coordinator in the country, told Reuters.
The fighting has also exacerbated ethnic tensions between Kiir's Dinka people and Machar's Nuer. After the rebels seized Bentiu, Dinka residents of Bor town in Jonglei state attacked a United Nations base sheltering about 5,000 people, mostly Nuer, there on Thursday.
Joe Contreras, acting spokesman for the UN mission in the country (UNMISS), said 48 dead bodies were found inside the base, while South Sudanese authorities said they recovered 10 more bodies near the compound, bringing the total killed to 58.
Another 98 people, including two Indian security personnel for UNMISS, were injured in the attack, Contreras told Reuters. (Writing by Duncan Miriri; Editing by Tom Heneghan)