Sudan's President Omar al-Bashir has travelled to Juba for talks with President Salva Kiir over the conflict in South Sudan, state media say.
Fighting is continuing in South Sudan as the warring parties meet in Ethiopia to try to agree a ceasefire. So far no substantive progress has been made.
The conflict pits supporters of Mr Kiir against rebels led by his sacked deputy, Riek Machar.
At least 1,000 people have been killed since violence erupted on 15 December.
The unrest started after South Sudan's president accused Mr Machar of attempting a coup - which he denies.
Nearly 200,000 people have been displaced in the conflict, which has taken on ethnic undertones. Mr Kiir is from the majority Dinka community and Mr Machar from the Nuer group.
State media in Sudan reported that President Bashir left for the South Sudanese capital, Juba, on Monday to discuss the crisis with Mr Kiir - accompanied by a team of senior ministers.
Earlier, the foreign ministry spokesman in Khartoum reaffirmed Sudan's wish to see "a continuation of the political process aimed at finding a peaceful resolution to the conflict in South Sudan", according to AFP news agency.
Sudan relies on revenue from oil transported through its southern neighbours' pipelines, which Khartoum fears will be disrupted by the fighting.
Up until Friday, the talks in the Ethiopian capital, Addis Ababa, were conducted by mediators. Now, teams representing the opposing factions in South Sudan are expected to negotiate face-to-face.
A preliminary meeting was held late on Saturday. Key issues are establishing a ceasefire, and the rebels' demand for the release of what they see as political prisoners.
But substantive talks failed to get under way on Sunday, delayed by disagreements over the agenda and - an official was quoted as saying - by "protocol issues".
International mediators may be losing patience with South Sudan's leaders whose delays are costing hundreds of lives, he says.
It is now hoped talks will begin in earnest on Monday.
Heavy fighting is continuing to the south of Bor, one of two cities held by rebels, says the BBC's Alastair Leithead, who is on the road between Juba and Bor.
The rebels are a former military division made up of thousands of men who switched sides, our correspondent says.
Until a ceasefire is agreed, fighting is expected to continue or even intensify, he adds.
On Sunday, a South Sudanese army general was killed when a government convoy was ambushed.
South Sudan is the world's newest state. It was formed in 2011, gaining independence from Sudan after decades of conflict.
The latest trouble has its roots in tensions that go back long before 2011.