JUBA/ADDIS ABABA - Gunshots rang out in South Sudan's capital, Juba, for about an hour on Sunday as peace talks between rebels and the government to hammer out a ceasefire deal faced further delay in neighbouring Ethiopia.

The gunfire came from the direction of the military headquarters of the SPLA government forces, towards the northern outskirts of the city. It was not clear who was involved.

Three weeks of fighting, which began in Juba but spread beyond, often along ethnic faultlines, have killed more than a thousand people, forced a cut in oil output and left the world's newest state on the brink of civil war.

Juba has been largely calm since the early clashes, though there was also a brief gun fight on Saturday evening and residents talk of growing tensions.

"I saw a truck full of soldiers going along the Bilpam road. They were singing. About 20 minutes later the shooting started and people started running towards town," said Animu Afekuru, who lives in the neighbourhood.

Western and regional powers, many of which supported the negotiations that led to South Sudan's secession from Sudan in 2011, are pressing for a peace deal, fearing the latest fighting could destabilise east Africa.

The unrest pits President Salva Kiir's SPLA government forces against rebels loyal to former vice president Riek Machar.

Both warring factions have said they want peace and are committed to a ceasefire in principle, though neither has indicated when they would lay down their weapons.

But there is widespread scepticism in Juba, where residents are on edge amid rumours of a rebel advance on the city that lies on the banks of the White Nile.

"I fear for our country in the coming days," said 19-year-old Nyathok Khat. "The politicians don't care about the suffering of the people."

NO "DELAY GIMMICK"

Fighting also erupted outside the flashpoint town of Bor, capital of vast Jonglei state which has untapped oil reserves.

US Secretary of State John Kerry on Sunday voiced his support for the Addis Ababa peace talks and warned against the use of force by either side to gain the upper hand.

"The negotiations have to be serious. They cannot be a delay gimmick in order to continue the fighting and try to find advantage on the ground at the expense of the people of South Sudan," Kerry told reporters during a visit to Israel.

Rebel and government negotiators were supposed to sit down face-to-face for the first time on Sunday. But the rebel delegation and a Western diplomat told Reuters late in the evening there would be no meeting that day.

Kiir blamed his long term rival, whom he sacked in July, for starting the fighting in a bid to seize power. Machar dismissed the allegation but he has acknowledged leading soldiers battling the government.

A key stumbling block to the talks is what should happen to a number of political detainees allied to Machar who are accused of involvement in the plot.

The rebels have demanded their comrades' release - a call backed by the United States and European Union.

"This is a capital offence, it is a case of treason and we are expected as the government of the Republic of South Sudan to investigate within two, three days? This is out of the question," South Sudan's Information Minister Michael Makuei told reporters in Addis Ababa.

Several false starts have dampened hopes for a swift end to the fighting, which has driven more than 200,000 people from their homes. The United Nations is scrambling to raise money to provide food, clean water and shelter.

Sudan's state news service reported Sudanese President Omar Hassan al-Bashir would head to Juba on Monday to meet Kiir.