South Sudan's president has said groups within the country's security forces are plotting a coup, hours after heavy gunfire rang out in the capital Juba.
Salva Kiir said on Monday that a curfew had been imposed and that he would hold a news conference to speak about the attempted coup.
Riek Machar, who was fired as vice president in July along with other ministers, was arrested and the army said it was firmly in control.
Colonel Philip Aguer, the South Sudan military spokesman, said some military installations in Juba had come under attack from armed soldiers who have since been repulsed.
The officer gave no further details, saying an investigation was under way and that the situation was tense but not likely to deteriorate.
"So far the army is in full control of Juba,'' Aguer said.
An Associated Press reporter saw heavily armed soldiers patrolling the streets of Juba early on Monday amid sporadic gunfire emerging from Juba's main army barracks.
There has been political tension in the world's youngest nation since South Sudan President Salva Kiir sacked Riek Machar as his deputy in July.
Machar, who has expressed a willingness to contest the presidency in 2015, said after his sacking that if the country is to be united it cannot tolerate a "one man's rule". His sacking, part of a wider dismissal of the entire Cabinet by Kiir, had followed reports of a power struggle within the ruling party.
The local Sudan Tribune newspaper reported on its website that clashes erupted late on Sunday between members of the presidential guard.
The fighting seemed to pit soldiers from Kiir's Dinka tribe against those from the Nuer tribe of Machar.
In a message to US citizens on Monday, the US embassy in Juba said it had received "reports from multiple reliable sources of ongoing security incidents and sporadic gunfire in multiple locations'' across Juba.
"The US Embassy has not been able to confirm that gunfire and insecurity have fully ceased,'' the message said.
"The embassy recommends that all US citizens exercise extra caution at all times. The US Embassy will continue to closely monitor the security environment in South Sudan, with particular attention to Juba city and its immediate surroundings, and will advise US citizens further if the security situation changes."
Hilde Johnson, special representative of the United Nations secretary-general for South Sudan, said in a statement that the UN mission in Juba was "deeply concerned" over the fighting that broke out late on Sunday and which continued on Monday.
"As the Special Representative of the Secretary General I urge all parties in the fighting to cease hostilities immediately and exercise restraint,'' the statement sad.
"I have been in touch regularly with the key leaders, including at the highest levels to call for calm.''