Heavy fighting raged in South Sudan's oil-producing north Thursday, officials said, although neighbours Kenya and Ethiopia claimed "good progress" in efforts to broker an end to the civil war there.

Army spokesman Philip Aguer said troops loyal to President Salva Kiir were battling forces allied to former vice president Riek Machar inside the town of Malakal, capital of Upper Nile state.

He also said troops were preparing an offensive against Bentiu, the main town in oil-rich Unity State, to follow on from their recapture of Bor, another state capital that had fallen into rebel hands during the nearly two weeks of clashes.

"There is fighting in Malakal. Our forces are in the northern part of Malakal and the rebels are on the southern part. We will flush them out of Malakal," Sudan People's Liberation Army (SPLA) spokesman Aguer told AFP.

"The rebels are still controlling Bentiu but the SPLA is planning to retake Bentiu soon," he added.

The violence in South Sudan, a fledgling oil producer which won independence from Sudan just two years ago, has left thousands dead, according to the United Nations.

Ten of thousands of civilians have also sought protection at UN bases amid a wave of ethnic violence pitting members of Kiir's Dinka tribe against Machar's Nuer clansmen.

The UN Security Council voted Tuesday to send nearly 6,000 extra soldiers and police to South Sudan, nearly doubling the UNMISS force to 12,500 troops and 1,323 civilian police.

Amid reports of bodies piled in mass graves and witness testimonies of massacres and summary executions and rapes, UN Secretary General Ban Ki-moon has promised those responsible would be "held accountable".

Crude prices have also edged higher because of the fighting as oil production, which accounts for more than 95 percent of South Sudan's economy, was dented by the violence and oil workers evacuated.

Kenyan President Uhuru Kenyatta and Ethiopian Prime Minister Hailemariam Desalegn flew into Juba on Thursday for talks with President Kiir, the latest in a line of peace brokers who have made the trip since the fighting began on December 15.

'Good progress' for peace mission

Ethiopian Foreign Minister Tedros Adhanom Ghebreyesus said "good progress" was made in the talks, with further talks of the Inter-Governmental Authority on Development(IGAD) -- a regional grouping -- scheduled to take place in Nairobi on Friday.

He said among the issues discussed was the possible release of the 11 senior figures close to Machar who were arrested at the outbreak of violence.

"The release of the detainees is part of what we discussed. The release of the detainees could be part of the solution," he said.

The fighting started after Kiir accused Machar, whom he sacked in July, of attempting a coup.

Machar denied this, and said the president was exploiting a clash between members of the army as a pretext to carry out a purge.

Although Kiir and Machar say they are open to peace talks, fighting has spread to half of the country's 10 states.

The battles have been intense. An AFP correspondent who visited the recaptured town of Bor on Wednesday said bodies littered the streets and stores were looted.

The UN said aid agencies need $166 million (121 million euros) over the next three months to distribute food, manage camps for the 90,000 displaced and provide health and sanitation.

The first reinforcements to the UN peacekeeping mission in South Sudan are expected to arrive within two days, the world body's special envoy to the country, Hilde Johnson, said.

Both manpower and equipment would be sent, Johnson said, without offering further details. Helicopters and a military transport plane are eventually expected to be deployed to South Sudan.

UN rights chief Navi Pillay said a mass grave had been found in rebel-held Bentiu and cited reports of at least two more in Juba, the capital. Around 15 bodies were found in one site in Bentiu, and another 20 bodies at a nearby river, she said.

The UN mission in Juba was more cautious, confirming 15 killed but saying it was still "investigating reports of such atrocities".

Nearly 100 US troops are on the ground in South Sudan, and the US military has deployed a "platoon-sized" Marine contingent to neighbouring Uganda. Four US troops were wounded on Saturday when their aircraft was shot at during an evacuation operation.

The United States was instrumental in South Sudan's independence from the north.

Washington on Thursday reaffirmed its vow to cut aid to South Sudan if Kiir's government is overthrown in a coup.

China meanwhile said it would soon dispatch its special envoy for African affairs to South Sudan to make contact with all sides and help the situation quickly return to stability.

burs-sas/gk/rmb