The United Nations said Monday it was "deeply concerned" over fighting in South Sudan's capital Juba and that it was in contact with the nation's leaders to call for calm.
"As the Special Representative of the Secretary General I urge 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," the Special Representative of the Secretary-General, Hilde Johnson, said in a statement.
Heavy gunfire erupted overnight, diplomats said, amid mounting political tensions in the newly-independent nation.
Diplomats and security sources said the fighting appeared to have broken out in a barracks close to the city centre shortly before midnight and involved the use of heavy machine guns and mortars.
Political tensions have been high in South Sudan in recent weeks, and earlier this month key leaders of the ruling party - the Sudan People's Liberation Movement (SPLM) - made a public challenge to President Salva Kiir and accused him of "dictatorial" behaviour. (READ: Kiir names new ministers, governor)
South Sudan's Information Minister Michael Makuei Lueth refused to comment on the unrest, but signalled the president was still in charge.
"The president is going to speak soon. I can't say anything until he speaks," he said.
A diplomat in the city described the situation as "quite confused", but added that relative calm returned to the city at 9:00 am with troops posted at major intersections.
Statements from the US and British embassies in Juba urged their nationals to avoid unnecessary movements.
The US embassy said there were "reports from multiple reliable sources of ongoing security incidents and sporadic gunfire in multiple locations across Juba".
The British embassy, through its twitter account, advised British nationals to "stay at home and avoid movement unless necessary".
A security source said the fighting broke out shortly before midnight Sunday, apparently between rival factions in South Sudan's armed forces.
South Sudan won its independence in 2011 after its people voted overwhelmingly in a referendum to split from the north and form a new nation.
Since independence, Kiir has struggled to stem rampant corruption and quell rebellion and conflict in the grossly impoverished but oil-rich nation, left devastated by decades of war.
The dissident SPLM group is led by powerful politician Riek Machar, a charismatic but controversial leader who fought on both sides of Sudan's brutal 1983-2005 civil war, and who was sacked as vice-president in July. (READ: S. Sudan president suspends vice president, cabinet)
It also includes Rebecca Garang, the widow of South Sudan's founding father John Garang. The challenge to Kiir earlier this month made public the bitter divisions within the former rebel movement turned political party.