JUBA - South Sudan's president said Monday he had defeated a coup attempt by a political rival following a night of fighting in the capital Juba.
"This was an attempted coup," President Salva Kiir told reporters.
He also said in a statement that the "government is in full control of the security situation in Juba. The attackers fled and your forces are pursuing them."
He also said an overnight curfew would be imposed from 6pm Monday to 6am Tuesday, and then remain in force until further notice.
The president blamed the fighting on "a group of soldiers allied to the former vice president Riek Machar and his group".
Machar leads a dissident group within South Sudan's ruling party -- the Sudan People's Liberation Movement (SPLM) -- and was sacked as vice-president in July.
Heavy fighting broke out, officials said Monday, amid mounting political tensions in the newly-independent nation.
Diplomats and security sources said the fighting appeared to have erupted in a barracks close to the city centre shortly before midnight and involved the use of heavy machine guns and mortars. The sporadic fighting raged throughout the night before calm was restored on Monday morning.
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.
But political tensions have been high 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.
SPLA army spokesman Phil Aguer told local radio that troops loyal to the president were "establishing the identity of those who started the shooting."
"The army is in control of the situation... the army is pursuing the attackers," he said.
South Sudan's Information Minister Michael Makuei Lueth 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 (0600 GMT) 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".
In a statement, the United Nations said it was "deeply concerned" over the fighting and that it was in contact with South Sudan's leadership.
"As the Special Representative of the Secretary General I urge all parties in the fighting to cease hostilities immediately and exercise restraint," UN Special Representative Hilde Johnson said in a statement.
"I have been in touch regularly with the key leaders, including at the highest levels to call for calm," she added.
A security source said the fighting broke out shortly before midnight Sunday, apparently between rival factions in South Sudan's armed forces, and raged until the early hours of the morning.
Civil aviation and airline sources also said that Juba airport had been shut indefinitely.
Since independence, President 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.
The dissident group 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.