Kenyan foreign ministry official Karanja Kibicho also confirmed Wednesday there will be a summit of the Intergovernmental Authority on Development (IGAD), an East African bloc, in Nairobi on Friday, dpa reported.
"We will expect President (Salva) Kiir to be there as this is about him," said Kibicho.
The finer details of both meetings were still being organized, officials said.
The African Union and IGAD released a joint statement, urging dialogue and an end to conflict.
"The AU and IGAD are profoundly concerned by reports of the mobilization of tribal militias in South Sudan, which threaten to further escalate the conflict and transform it into an exceptionally destructive inter-ethnic violence that would put in danger the very existence of South Sudan," the groups warned.
South Sudan President Kiir has expressed willingness to negotiate an end to the fighting, now its second week, which has killed many hundreds.
His rival, former vice president Riek Machar, has also said he is ready for talks, but is demanding that Kiir step down, accusing him of being dictatorial.
Hilde Johnson, the top UN official in South Sudan, said she has been in touch with both leaders.
"They do not want a further escalation of violence. And they have both said they want to talk, to have a dialogue and to give peace a chance," Johnson said.
"The leaders of the region, of IGAD, are now engaged in active efforts to bring the two parties to the table. We hope progress can be made very soon," according to Johnson, who warned that "the country is at a crossroads.
UN Secretary General Ban Ki-moon issued a video and radio message Wednesday vowing the UN would stand with the people of South Sudan and pointing to the targeting of civilians based on ethnicity as a "grave violation of human rights."
"South Sudan is under threat - but South Sudan is not alone," he said, calling on leaders to settle their differences peacefully and warning those responsible for crimes would be held responsible.
The European Union was also sending special envoy Alexander Rondos to South Sudan to support a political solution to the conflict, Maja Kocijancic, spokeswoman for EU foreign policy chief Catherine Ashton, announced via Twitter on Wednesday.
Ashton called on the parties in the conflict to lay down their weapons, saying she was "deeply concerned" over the worsening situation in South Sudan, she said.
Meanwhile, the United Nations Mission in South Sudan (UNMISS) denied the existence of a mass grave in Bentiu, the capital of Unity state.
"Latest reports suggest that this is an inflation of a skirmish resulting in about 15 fatalities, with details to be confirmed. UNMISS is therefore not in a position to verify the existence of such a mass grave in Unity State thus far," according to a statement.
On Tuesday, UN High Commissioner for Human Rights Navi Pillay said her office had discovered a mass grave in Bentiu and received reports of at least two other mass graves in Juba, the country's capital.
UNMISS said it was investigating all reports of atrocities.
At least three out of ten states in South Sudan are experiencing violence. The conflict has taken on ethnic dimensions. Kiir is a Dinka and Machar is from the Nuer people.
The UN Security Council unanimously adopted a resolution on Tuesday for 5,500 peacekeepers to bolster UNMISS, which already has about 7,000 military members. An additional 1,300 civilian police were to join the mission.