The government has been under diplomatic pressure to end the violence, which erupted in mid-December and has killed at least 1,000 people, the BBC reported.
The clashes between the Dinka and Nuer ethnic groups have also displaced 200,000 people.
Peace talks began on Sunday in Ethiopia.
Chief rebel negotiator Taban Deng said that the rebel groups are confident that a full reconciliation can be achieved.
On Monday, the South Sudanese army reached a ceasefire agreement with the rebel group led by David Yau Yau in the state of Jonglei, the Sudan Tribune reported.
The agreement will give the two sides a chance to hold peace talks, the army spokesman Phillip Aguer said.
It remains unclear if the rebels will respond to the ceasefire agreement.