The UN has called for an immediate ceasefire in South Sudan, after more than three weeks of violence that killed at least 1,000, and displaced close to 200,000 people.

Parts of the country are in a state of emergency as the warring factions were expected to begin talks in the Ethiopian capital Addis Ababa on Thursday.

"We have seen terrible acts of violence in the past two weeks, there has been killings and brutality, grave human rights violations and atrocities committed," Hilde Johnson, UN special representative to South Sudan, told Al Jazeera.

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Johnson urged the government and the rebels to stop the fighting as negotiations were about to get underway.  

The UN has also joined the African Union in expressing support for Ethiopia to broker peace talks.

But Johannes Musa, a member of the negotiating team for former Vice President Riek Machar, told Al Jazeera that there are significant problems to be overcome, and that the rebels will not lay down their arms unilaterally.

"We did not refuse a ceasefire," Musa said. " But we put out some conditions.

"The government may not commit itself to mutual ceasefire, that will not be monitored."

Musa said that in the absence of ceasefire monitors, the government could launch raid in areas already controlled by the rebels.

The South Sudan government has also refused to call it a ceasefire, saying negotiators must first agree on "mechanisms" for talks to move forward.

Late on Wednesday, South Sudanese President Salva Kiir declared a state of emergency in Unity and Jonglei states, where government troops and rebel forces loyal to Machar, have been engaged in fighting.

Ethiopian mediation

Delegations from South Sudan's warring factions were expected to meet for the first time on Thursday in the Ethiopian capital Addis Ababa.

Some Western diplomats were also present as mediators waited for the arrival of the South Sudan government's delegation.

Ethiopia is playing a leading role in trying to get the two sides to negotiate a peace deal.

Government troops are heading to Juba, capital of Jonglei state

But those efforts have been overshadowed by persistent violence in South Sudan since mid-December.

Under a regional bloc known as IGAD, East African countries have urged Machar and Kiir to negotiate an end to violence that raised fears of civil war in the world's newest country.

The fighting in South Sudan has exposed ethnic rivalry between the country's two largest ethnic groups, the Dinka of Kiir and the Nuer of Machar.

The UN has said there is mounting evidence that people were targeted for their ethnicity.

Kiir insists the fighting was sparked by a coup attempt mounted by soldiers loyal to Machar on Dec.15 in the capital, Juba.

But that account has been disputed by some officials of the ruling party, who said the violence began when presidential guards from the Dinka group tried to disarm their Nuer colleagues.

From there, violence spread across the country, with forces loyal to Machar defecting and seizing territory from loyalist forces.

South Sudan has been plagued by ethnic tension and a power struggle within the ruling party that escalated after Kiir dismissed Machar as his vice president in July.

Machar has criticised Kiir as a dictator and said he will contest the 2015 presidential election.

In an interview with Al Jazeera on Thursday, Aly Verjee, a political analyst, said the conflict was born out of a political dispute among the elites in the ruling party.

"There are ethnic overtones, unfortunately, that have developed over the course of the last few weeks," Verjee, a senior researcher at the Rift Valley Institute in Kenya, said. "But it is still primarily a political dispute."

South Sudan peacefully broke away from Sudan in 2011 following a 2005 peace deal. Before that, the south fought decades of war with Sudan.

Source: Al Jazeera and agencies