East Africa's leaders have said they will not accept a violent overthrow of the government in South Sudan, where more than 1,000 people are believed to have died in recent clashes.
Kenya's president also called for an immediate end to the fighting and for talks with both sides.
The UN is sending extra peacekeepers to the world's newest country.
The clashes come amid a power struggle between President Salva Kiir and his former deputy, Riek Machar.
There are reports of mass killings along ethnic lines, targeting both members of Mr Kiir's Dinka ethnic group and Mr Machar's Nuer community.
The fighting has forced more than 100,000 to flee their homes, with about 60,000 seeking refuge at UN compounds across the country, UN officials say.
'Give peace chance'
East African regional leaders from the eight-member bloc, known as Igad, are meeting in Kenya's capital Nairobi after the leaders of Kenya and Ethiopia met President Kiir in South Sudan's capital Juba on Thursday.
However, BBC South Sudan analyst James Copnall says there is no sign of the key step in solving this crisis - direct talks between the two protagonists.
President Kiir is the only Igad leader not attending the Nairobi talks but a Kenyan official sought to downplay his absence.
"Kiir is not coming because [Ethiopian Prime Minister Hailemariam] Desalegn and [Kenyan President Uhuru] Kenyatta met him yesterday in Juba. Everything they needed to discuss was discussed," a State House official told the Reuters news agency.
He is being represented by senior officials.
China, which buys most of South Sudan's oil, has also sent an envoy to the region to try to negotiate an end to the fighting.
"Let it be known that we in Igad will not accept the unconstitutional overthrow of a duly and democratically elected government in South Sudan," President Kenyatta said at the start of the talks.
"Violence has never provided optimum solutions - violence begets more violence," he said.
The Kenyan leader said the violence must be stopped immediately.
"We have a very small window of opportunity to secure peace, which we urge all stakeholders to seize, including [Riek] Machar," he said.
He said two peace envoys should be appointed to talk to both sides and suggested that one of them be Gen Lazaro Sumbeiywo - who helped broker the 2005 Sudan peace deal which led to the South's independence in 2011.
Rebecca Garang, widow of John Garang, who led south Sudanese rebel forces against Khartoum for many years, told the BBC's Newsday programme she was "optimistic" about the Nairobi talks and welcomed the regional intervention.
She said her late husband would not have been happy with the fighting in the world's newest country.
"Those people who have died for the cause of this country would not be happy with what we are doing today," she said.
The head of the UN mission in South Sudan, Hilde Johnson, on Thursday called on the country's political leaders "to give peace a chance".
"The nation that [was] painstakingly built over decades of conflict and strife is at stake," she said, speaking via a video link from Juba.
The UN Security Council has voted to almost double the number of peacekeepers to 12,500.
Rebel forces have seized two state capitals - Bor and Bentiu - and say they have captured a third, Malakal, although the army disputes this.
The fighting is also affecting oil production, which accounts for 98% of government revenue.
The violence erupted after President Kiir accused the former vice-president, who had been sacked in July, of plotting a coup.
Mr Machar denied any involvement and said the accusations were an attempt by Mr Kiir to get rid of his political rivals.
The fighting quickly spread to half of Sudan's 10 states.
South Sudan has been beset by a series of rebellions since becoming independent from Sudan in 2011.