Peace talks between South Sudan's government and rebels started in Ethiopia on Monday, as key power China added its weight to efforts to end weeks of fighting in the world's youngest nation.

Sudan meanwhile said that it and South Sudan had agreed during a visit to Juba by Sudanese President Omar al-Bashir to consider setting up a joint force to protect vital oilfields.

Ethiopian government spokesman Getachew Reda, whose government has spent days trying to get the two sides into the same room, told AFP that formal negotiations on a possible ceasefire had finally started in an Addis Ababa luxury hotel -- even as fighting continued to rage back in South Sudan.

Chinese Foreign Minister Wang Yi, speaking at the start of a four-nation Africa visit, said Beijing was also trying to push for peace and was actively engaged in mediation efforts. China has invested heavily in the country's oil sector and buys most of its crude output.

"China is a permanent member of the United Nations Security Council, so we are paying close attention to the evolving situation in South Sudan," Wang told reporters.

"We have been making mediation efforts, and the Chinese government special representative for African affairs is visiting the region and has had meetings with both sides," he said, offering to personally "directly engage" with the two sides while in Addis Ababa.

A surge in diplomatic efforts also saw Sudan's President Bashir jet into South Sudan's capital Juba for talks with his counterpart Salva Kiir during which he stressed Khartoum's support for "a peaceful resolution to the conflict".

Sudan's foreign minister Ali Ahmed Karti also said the two are "in consultations about the deployment of a mixed force to protect the oilfields in the South."

But fighting has continued to rage, with both sides vowing to step up their offensives across the country -- which has been teetering on the brink of all-out civil war less than three years after gaining independence from Khartoum.

Heavy fighting has been reported in oil-producing Unity and Upper Nile states in the north, and in particular near rebel-held Bor, capital of Jonglei State just north of the capital. Army spokesman Philip Aguer said it was only a "matter of time" before Bor was recaptured.

The conflict in South Sudan erupted on December 15, pitting army units loyal to Kiir against a loose alliance of ethnic militia forces and mutinous army commanders nominally headed by Riek Machar, a former vice president who was sacked last July.

Machar denies allegations that he started the conflict by attempting a coup, and in turn accuses the president of orchestrating a violent purge.

After the opening talks delegates insisted they wanted peace.

"We are here to assure you that we have started the process and we are optimistic that we will end it peacefully," government official Makuei Luoth said, while top rebel delegate Taban Deng said his team has "been working tirelessly to bring back peace."

Civilians suffering

UN officials say they believe thousands of people have already been killed, and both sides are alleged to have committed atrocities. UN peacekeeping bases have also been overwhelmed with tens of thousands civilians seeking shelter, many of them fleeing ethnic violence between Kiir's Dinka community and Machar's Nuer tribe.

"We're very concerned about the effects on the civilian population," the president of the International Committee of the Red Cross (ICRC), Peter Maurer, also said on the start of a visit to the country.

He said the Geneva-based organisation was "particularly alarmed by violence directed against civilians and against people no longer taking part in the hostilities" -- signalling that a reported wave of atrocities was ongoing.

British aid group Oxfam also reminded delegates at the Addis Ababa peace talks of their "duty to their citizens to reach a swift and peaceful resolution to the conflict".

"Thousands of families already living in extreme poverty have been pushed from their homes and cut off from what they need to survive," Oxfam's Desire Assogbavi said.

A top rebel delegate at the talks, however, indicated that finding a quick resolution to the conflict would be difficult.

"I am optimistic. Our delegation is going in with an open mind," rebel delegate Mabior Garang said, but added the rebels were "suspicious of the sincerity of the government."

"They keep shifting the goalposts and are adamant on not releasing detainees, but we should first get to the table and discuss a cessation of hostilities," he added.

A key sticking point has been rebel and international demands that the South Sudanese government release 11 officials close to Machar so they can participate in the talks, which diplomats hope will secure a truce as well as viable ceasefire-monitoring mechanisms.

The South Sudanese government, however, has repeated that the rebel suspects would not be freed and should face justice.

Secretary of State John Kerry of the United States, which was instrumental in helping South Sudan win independence, has urged rival southern factions not to use the Addis talks to buy time.

str-burs-sas/cc