Sudan's President Omar al-Bashir arrived Monday in Juba calling for an end to three weeks of fighting in South Sudan as mediators struggled to get peace negotiations under way in neighbouring Ethiopia.
There were reports meanwhile of ongoing fierce clashes near the rebel-held town of Bor, situated 200 kilometres (130 miles) north of the capital Juba, with South Sudan's army pouring in reinforcements in a bid to recapture the area.
"There should be peace and security in South Sudan," Bashir said as he visited Juba for talks with his counterpart President Salva Kiir.
"We come so that we can bring peace to South Sudan, to our brothers and sisters in South Sudan. Our relationship is very important," Bashir told reporters.
South Sudan won independence from Khartoum in 2011 after decades of war, but the north remains a key player -- serving as the export route for the South's oil.
Peace talks, brokered by the East African regional bloc IGAD and aimed at securing an elusive ceasefire, were set to start in Addis Ababa in the afternoon, Ethiopian government spokesman Getachew Reda said.
Despite movement on the diplomatic front, the fighting continued.
Army spokesman Philip Aguer said on Sunday it was only a "matter of time" before Bor was retaken, and said government forces were also on the offensive in the oil-producing Unity and Upper Nile states in the north.
The conflict in South Sudan erupted on 15 December, 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.
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 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, 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, aimed at putting in place a truce as well as ceasefire-monitoring mechanisms.
The South Sudanese government, however, has repeated that the rebel suspects would not be freed and should face justice.
US Secretary of State John Kerry has urged rival southern factions not to use the Addis talks for buying time.
"Negotiations have to be serious, they cannot be a delay, (a) gimmick in order to continue the fighting and try to find advantage on the ground at the expense of the people of South Sudan," he said on Sunday.
The United States, which was instrumental in helping South Sudan win independence, urged South Sudan's government to "release political detainees immediately".