South Sudan's army (SPLA) said it was advancing on two rebel-controlled towns on Thursday as both sides gathered in Ethiopia for peace talks to end three weeks of violence that has pushed the world's youngest nation towards civil war.
Both sides have agreed in principle to a ceasefire but neither has indicated when the fighting, which has killed more than 1,000 people and displaced nearly 200,000, will stop.
South Sudanese President Salva Kiir declared a state of emergency late on Wednesday in Unity state and Jonglei, whose respective provincial capitals of Bentiu and Bor are in the hands of militia loyal to former vice president Riek Machar.
A rebel spokesman in Unity dismissed the SPLA's comments on its advance as lies and said South Sudan's army and the national government in the capital Juba had resorted to a "war of allegations" before peace negotiations could get underway.
International pressure for a peace deal is mounting. Neighbouring countries mediating between the two warring sides have warned that continued fighting could scupper talks.
"We are advancing to Bor because these people want to come to Juba," SPLA Chief of Staff James Hoth Mai told reporters in the capital. "We don't yet have a ceasefire and we don't want them to come and get us."
Bor lies 190 kilometres (118 miles) north of Juba by road. Analysts say control of Bor hands the rebels a territorial base relatively close to Juba, strengthening their negotiating hand.
Mai said SPLA troops were also approaching Bentiu after seizing the nearby town of Mayom on Wednesday. A rebel spokesman in Bentiu said Mayom remained in rebel hands, a comment backed up by a spokesman for the United Nations mission in South Sudan.