South Sudan capital Juba hit by gunfire South Sudan President Salva Kiir (R) and former deputy Riek Machar (July 2013) President Salva Kiir (R) sacked Riek Machar and his entire cabinet earlier this year
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Heavy gunfire and explosions have been heard throughout the night in the South Sudanese capital Juba.
Local media said the fighting was between rival factions of the presidential guard and focused around their military barracks.
Heavily armed troops are now patrolling Juba, and army spokesman Col Phillip Aguer told the BBC the military was in full control.
The UN expressed concern and appealed for all sides to show restraint.
President Salva Kiir is expected to make a statement shortly.
South Sudan formally split from Sudan in 2011, after decades of conflict. But the oil-rich country is ethnically and politically divided, with many armed groups active.
Tensions have been particularly high since President Kiir dismissed his entire cabinet, including his deputy Riek Machar, in July in an apparent power struggle.
Mr Machar had indicated he planned to contest the presidential elections in 2015.
Mr Kiir is from the Dinka community, which is the largest in South Sudan, while Mr Machar is from the Nuer, the second-largest. Some Nuer have complained about Dinka political domination.
The fighting in Juba reportedly broke out overnight, and intensified in the early morning.
The Paris-based Sudan Tribune said the clashes began when one mostly Nuer unit of the presidential guard became suspicious of deployments of a group of mainly Dinka guards.
There were reports of continuous gunfire and the sound of explosions.
State TV channel SSTV was off air and the city's airport has been closed.
The situation had reportedly calmed by mid-morning, but heavily armed troops were seen on the capital's streets.
One resident who lives near the presidential guard barracks told the BBC that many people had sought refuge at a Catholic church.
Col Aguer said the army was "establishing the facts about the identity of those who started the fighting".
"The military intelligence is gathering information. As soon as the situation is cleared, the government will come up and the army will make a statement about what it was," he said.
Hilde Johnson, the UN's special representative in South Sudan, said she was "deeply concerned" about the fighting and urged "all parties in the fighting to cease hostilities immediately and exercise restraint".
"I have been in touch regularly with the key leaders, including at the highest levels to call for calm," she said.
The UN mission in Juba said earlier that staff there were under lockdown.
The UK and US embassies in Juba urged their citizens via Twitter to stay indoors and exercise caution.
The US statement to citizens said it had suspended all routine services amid "reports from multiple reliable sources of ongoing security incidents and sporadic gunfire in multiple locations across Juba".
The US embassy also denied rumours that Mr Machar had taken shelter there.
In a second statement, the US said embassy staff had spoken to a range of officials and concerned parties "in order to urge calm, restraint, and a settling of differences through a peaceful political means rather than through violence".
Sudan: A country divided
Oil fieldsGeographyEthnic groupsInfant mortalityWater & sanitationEducationFood insecurity Show regions
Map showing position of oilfileds in Sudan, source: Drilling info international
Both Sudan and the South are reliant on their oil revenues, which account for 98% of South Sudan's budget. But the two countries cannot agree how to divide the oil wealth of the former united state. Some 75% of the oil lies in the South but all the pipelines run north. It is feared that disputes over oil could lead the two neighbours to return to war.
Satellite image showing geography of Sudan, source: Nasa Although they were united for many years, the two Sudans were always very different. The great divide is visible even from space, as this Nasa satellite image shows. The northern states are a blanket of desert, broken only by the fertile Nile corridor. South Sudan is covered by green swathes of grassland, swamps and tropical forest.
Map showing Ethnicity of Sudan, source: Sudan's arid north is mainly home to Arabic-speaking Muslims. But in South Sudan there is no dominant culture. The Dinkas and the Nuers are the largest of more than 200 ethnic groups, each with its own languages and traditional beliefs, alongside Christianity and Islam.
Map showing infant Mortality in Sudan, source: Sudan household health survey 2006 The health inequalities in Sudan are illustrated by infant mortality rates. In South Sudan, one in 10 children die before their first birthday. Whereas in the more developed northern states, such as Gezira and White Nile, half of those children would be expected to survive.
Map showing percentage of households using improved water and sanitation in Sudan, source: Sudan household health survey 2006 The gulf in water resources between north and south is stark. In Khartoum, River Nile, and Gezira states, two-thirds of people have access to piped drinking water and pit latrines. In the south, boreholes and unprotected wells are the main drinking sources. More than 80% of southerners have no toilet facilities whatsoever.
Map showing percentage of who complete primary school education in Sudan, source: Sudan household health survey 2006 Throughout the two Sudan's, access to primary school education is strongly linked to household earnings. In the poorest parts of the south, less than 1% of children finish primary school. Whereas in the wealthier north, up to 50% of children complete primary level education.
Map showing percentage of households with poor food consumption in Sudan, source: Sudan household health survey 2006 Conflict and poverty are the main causes of food insecurity in both countries. The residents of war-affected Darfur and South Sudan are still greatly dependent on food aid. Far more than in northern states, which tend to be wealthier, more urbanized and less reliant on agriculture