* South Sudan officials says airport to re-open
* Western nations look at pulling out citizens
* President Kiir calls fighting a coup attempt
* Clashes revive memories of bloody ethnic rifts in 1990s (Adds residents reporting tense calm in Juba, reports of fighting outside of Juba, analyst comment)
By Andrew Green
JUBA, Dec 18 (Reuters) - The United States will start evacuating non-essential embassy staff and citizens from South Sudan on Wednesday after army factions clashed, killing up to 500 people and raising fears of a broader civil conflict in the two-year-old nation.
Gunbattles erupted on Sunday night between army factions, split along ethnic lines, focused around two army barracks next to the capital in what the government called a coup attempt.
Residents said Juba was calm on Wednesday after sporadic gunfire overnight. Traffic was slowly returning to the streets and officials said the airport would re-open on Wednesday.
The government said it arrested 10 people, including seven former ministers, over the "foiled coup" and wanted to question several others, including former Vice President Riek Machar.
The United States, which has urged its citizens to leave South Sudan, said it would start evacuating its people from Juba airport on flights organised by the U.S. State Department.
Britain said it was gathering the names of any citizens who wanted to leave and residents said other Western nations were expected to follow. Many aid workers live and work in Juba.
Diplomats said the United Nations had reports of between 400 and 500 people killed and up to 800 wounded in the oil producing nation, which has been struggled to build a functioning state.
"Most people are scared they might be confronted with a mob or see dead bodies," said one aid worker in Juba, adding that the city was calmer on Wednesday morning, after residents awoke to heavy gunfire and artillery blasts on Monday and Tuesday.
President Salva Kiir blamed the clashes on backers of Machar, who he sacked in July. After his sacking, Machar said he would run for president and accused Kiir of being dictatorial.
Kiir, who before the clashes said his rivals were reviving rifts that led to bloodshed in the 1990s, has faced public criticism for doing little to improve life in one of Africa's poorest nations that won independence from Sudan in 2011.
"Salva must recognise that the charge of his being 'dictatorial' has taken deep hold, and he must do what is necessary to shed the label as much as possible," South Sudan expert Eric Reeves wrote in an assessment of the violence.
Kiir's dominant Dinka group and Machar's Nuer have fought before. But analysts say divisions in South Sudan run deep with opposing factions running broadly along ethnic lines emerging in the army, probably beyond either leaders' control.
"There are a number of urgent steps he (Kiir) must take to secure the confidence of the international community, and to put the Nuer community at ease in the Juba area and elsewhere," Reeves said. "He must ensure that fighting does not spread."
Conflict between southerners could hurt vital aid flows and might be exploited by Sudan's government, analysts said.
South Sudan has frequently been caught up in political rows with Sudan, leading to a 15-month shutdown of oil production and exports through a pipeline running through Sudan, cutting off the young nation's almost sole source or revenue.
Though fighting has centred around the capital, there have been some reports of clashes elsewhere.
Hussein Maar, deputy governor of Jonglei State, north of Juba, reported fighting at two barracks a few kilometres from the town of Bor, the place where in 1991 a faction of soldiers loyal to Machar massacred hundreds of Dinka.
Majok Guangdong Thiep, South Sudan's ambassador to Kenya, told Kenyan television the airport would re-open on Wednesday. He said the situation in Juba was under control.
However, an official at Kenya airline Fly540 said a flight due to depart at 8 a.m. (0500 GMT) from Nairobi for Juba did not leave because the South Sudanese airport was not open.
"Planes due to take off for Juba are empty as people don't want to go there. Even the Sudanese don't want to go home," said the Fly540 official, who asked not to be identified. (Writing and additional reporting by Drazen Jorgic and Edmund Blair,; editing by Elizabeth Piper)