Juba - South Sudan's President Salva Kiir has imposed a state of emergency in two states, the government said on its Twitter feed.
The state of emergency declared on Wednesday will apply to Unity and Jonglei states where government forces are fighting rebels.
The government was sending thousands of troops to the city of Bor to recapture the capital of Jonglei from rebels, the broadcaster Al Jazeera reported. Kiir has put together a delegation for peace talks in Ethiopa with the rebels, who are led by his former vice president Riek Machar.
Thousands of people have fled there, according to the International Committee of the Red Cross (ICRC). They have crossed the Nile to take refuge in Awerial county in the neighbouring Lakes state. "The road to the river is lined with thousands of people, with others waiting for boats to carry them across," said Francois Moreillon, ICRC deputy head of delegation.
The ICRC said those fleeing needed urgent help. The organisation said its initial response would be to provide 30 000 people with basic items such as blankets, cooking kits, jerry cans and a two-week food supply.
The violence in South Sudan, the world's youngest nation, flared two weeks ago between troops loyal to Kiir and those supporting Machar, whom Kiir sacked in July. Fighting has spread to six out of 10 states, killing hundreds and displacing some 120 000 others.
That Kiir and Machar come from different ethnic groups and regions within South Sudan has caused further political ruptures. Kiir belongs to the Dinka, the largest group in South Sudan, and Machar is from the Nuer people.
Kiir and Machar agreed on New Year's Eve to begin peace talks that are due to begin in the next few days in the Ethiopian capital Addis Ababa. A ceasefire is one of the aims of the talks.