South Sudanese President Salva Kiir has declared a state of emergency in two states.
The move came as his negotiators prepared for peace talks with rebels to end more than two weeks of violence that has pushed the country towards civil war.
Mr Kiir called the emergency in Unity and Jonglei states, the two regions whose capitals are now controlled by rebel forces loyal to former vice president Riek Machar, who Mr Kiir has accused of plotting a coup.
Gunshots were heard near the presidential palace this evening.
A presidential spokesman said security forces most likely fired the shots at residents breaking a curfew.
Both sides are under mounting pressure from regional and Western powers to reach a deal to stop the bloodletting that has killed more than 1,000 people in the world's newest state and displaced nearly 200,000 more.
The White House has said it would deny support - vital in a country the size of France that still has hardly any infrastructure more than two years after secession - to any group that seizes power by force.
The rebel delegation earlier arrived in the Ethiopian capital Addis Ababa ready for the ceasefire talks. They said government negotiators had not yet arrived.
Both sides have agreed in principle to a ceasefire but neither has indicated when the fighting would stop and mediators are concerned that fighting around the flashpoint town of Bor will scupper the talks even before they begin.
South Sudan's defence minister earlier said government forces were battling rebel fighters 18km south of Bor, the capital of Jonglei state, which has untapped oil reserves and was the site of an ethnic massacre in 1991.
"There will have to be a fight because they want to defeatthe government forces," Defence Minister Kuol Manyang Juuk said.
Rebels loyal to Mr Machar seized control of Bor yesterday.
The Addis Ababa talks will focus on finding ways to roll out and monitor the ceasefire, the East African IGAD bloc that is mediating the talks said.
The clashes, which have spread to half the country's ten states have unsettled oil markets and raised fears of the conflict spilling over in an already fragile region.
"We don't want to expose the people of South Sudan to a senseless war," South Sudan's Foreign Minister Barnaba Marial Benjamin said on a government Twitter feed.
Mr Kiir has accused his long-term political rival Mr Machar, who he sacked in July, of starting the fighting in a bid to seize power.
Mr Machar has denied the charge, but he has taken to the bush and has acknowledged leading soldiers battling the government.
Clashes between soldiers erupted on 15 December in Juba.
The violence quickly spread, dividing the country along the ethnic lines of Mr Machar's Nuer group and Mr Kiir's Dinkas.