South Sudanese President Salva Kiir has declared a state of emergency in two states, according to the government's official Twitter account.

The decree issued on Wednesday covers Unity and Jonglei, where government troops and rebel forces loyal to Riek Machar, former vice president, have been engaged in fighting.

The declaration came as the rival factions were set to open peace talks in Ethiopia aimed at bringing an end to the nearly three-week-old civil war, despite reports of an imminent military showdown in Jonglei.

CEOAFRICA source said government and rebel negotiators arrived in the Ethiopian capital Addis Ababa on Wednesday.

Getachew Reda, Ethiopian government spokesman, said the talks would focus on "monitoring mechanisms for the ceasefire".

Following the fall of Bor, Jonglei's state capital, into the hands of the rebels on Tuesday, the government and rebels loyal to Machar agreed to meet for talks.

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

Meanwhile, the head of the UN peacekeeping mission in South Sudan has called for an end to fighting in the country, and for peace talks to be given a chance.

Hilde Johnson told newsmen the humanitarian situation was worsening.

At least 1,000 people have died since fighting erupted last month between supporters of President Salva Kiir and those of his sacked deputy Riek Machar.