South Sudanese soldiers patrol near an oil refinery in Melut, Upper Nile State, South Sudan
Juba (AFP) - South Sudan troops and rebels are fighting heavy battles close to vital oil production zones in the northeastern battleground state of Upper Nile, the United Nations said Wednesday.
Two mortar bombs hit the UN base in Melut, killing four civilians including a child, who were among more than 1,500 people sheltering there, the UN said. Eight others were wounded.
Melut lies some 35 kilometres (20 miles) west of the main oil production base at Palouch, which rebels are trying to capture.
The United Nations peacekeeping mission in South Sudan (UNMISS) said Wednesday it "strongly condemns the recent upsurge in fighting".
It was not possible to confirm rebel claims of fighting inside oil fields nor their reports of having killed scores of soldiers, which the government has dismissed as "lies".
The country's 17-month-long war broke out in December 2013 when President Salva Kiir accused his former deputy Riek Machar of attempting a coup, setting off a cycle of retaliatory killings across the country.
Rebel spokesman James Gadet Dak on Tuesday ordered oil companies to evacuate staff and said they were fighting to take the oil zones in Upper Nile to "deny Salva Kiir from using the oil revenues to perpetuate the war".
South Sudan, which won independence from Sudan in 2011 to become the world's youngest nation, once saw 95 percent of the government's budget come from oil revenues.
But oil production has slumped since war broke out, with many oil fields damaged in the fighting, badly denting the country's economy.
- Girls raped, boys abducted -
Oil companies include China National Petroleum Corp, India's Oil and Natural Gas Corp and Malaysia's Petronas.
Oil fields in neighbouring Unity state, where the government is advancing on a rebel enclave, have almost all stopped pumping due to fighting.
Information Minister Michael Makuei late Tuesday dismissed rebel claims of capturing key oil areas and said the production had not been affected.
Government troops launched an assault in late April, one of the heaviest offensives yet in the civil war, which has cut off over 650,000 from aid and seen gunmen raping, torching towns and looting relief supplies, according to the UN and aid agencies.
The UN children's agency this week said girls as young as seven had been raped or killed, boys as young as 10 had been killed and others had been mutilated or abducted by "armed groups aligned with" the army.
But rebels launched a major counter-attack, including an assault last week on Malakal, the state capital of Upper Nile and the gateway to the country's last remaining major oil fields.
"UNMISS is deeply concerned by the overall escalation of the conflict, and once again calls on the government and opposition leadership to immediately cease all hostilities and resume negotiations," the UN said.
A string of peace deals all collapsed within days. The war has left over half of the country's 12 million people in need of aid, with 2.5 million people facing severe food insecurity, according to the UN.