Thousands of people are feared dead in violence that began 11 days ago and has raised fears of a civil war.
President Salva Kiir was seen going into talks with Ethiopian PM Hailemariam Desalegn and Kenyan President Uhuru Kenyatta on Thursday.
Mr Kiir is involved in a power struggle with ex-deputy Riek Machar.
The fighting has exposed ethnic divisions in the world’s youngest state as the president is an ethnic Dinka, while Mr Machar represents the Nuer tribe.
UN officials believe thousands have been killed since violence erupted on 15 December, when clashes broke out at a meeting of the ruling Sudan People’s Liberation Movement (SPLM).
President Kiir accused the former vice-president, who had been sacked in July, of plotting a coup. Mr Machar denies trying to seize power.
Clashes have since spread to half of South Sudan’s 10 states.
Tens of thousands of people have fled to UN compounds across the oil-rich country. The UN Security Council has decided to boost the existing UN force (Unmiss) of around 7,000 peacekeepers and police to about 14,000.
In a Christmas Day message, UN Secretary General Ban Ki-moon said “South Sudan is not alone”.
President Kenyatta, current head of the East African Community, was met by senior government officials as he arrived in Juba shortly before 10:00 (07:00 GMT), Kenyan media reported.
Talks with Mr Kiir and the Ethiopian prime minister were then held behind closed doors, officials said.
The European Union is sending an envoy, Alex Rondos, to Juba to push for a negotiated solution. China said its special envoy to Africa would also travel to Juba “soon”.
Both the South Sudanese leader and Mr Machar have said they are willing to discuss an end to the crisis.
But the former vice-president has said his detained political allies must first be freed, while Mr Kiir says there should be no preconditions.
South Sudan’s Information Minister, Makuei Lueth, told the Associated Press on Thursday that no contact with Mr Machar had yet been established. “For us, we are not talking with him,” he said.
‘Sense of confidence’
After the violence broke out, forces loyal to Mr Machar seized the key towns of Bor, the capital of Jonglei state, and Bentiu, the main town in oil-producing Unity state.
But government forces recaptured Bor on Tuesday and are trying to retake Bentiu.
Unmiss spokesman Col Michael Chadwick told BBC News that the area around Bor was currently relatively stable.
Some 15,000 people had fled to the UN compound in Bor but up to 2,000 of them were now moving out as the situation had become safer, he said.
“This is giving us a sense of confidence that we’ll be able to establish more Unmiss flights and possibly more humanitarian flights.”
South Sudan has struggled to achieve a stable government since becoming independent from Sudan in 2011.
The independence referendum was intended to end a decade-long conflict, led by the SPLM, against the north. But the oil-rich country remains ethnically and politically divided, with many armed groups active.
After a peace deal was signed in 2005, the southern rebel group appointed Mr Machar as vice-president of the South Sudan regional government, a position he retained after independence in 2011 until he was dropped in July.