The US ordered its non-essential staff to leave the country because of the violence, warned US citizens not to visit and urged those already there to leave immediately.
The US embassy in Juba suspended "normal operations until further notice and cannot provide routine consular services to US citizens in South Sudan," the State Department said.
Various media reports said dozens of soldiers had been killed in two days of clashes between troops loyal to President Salva Kiir and those supporting his former deputy, Riek Machar, who was sacked in July.
The Associated Press later quoted UN diplomats as saying as many as 400 to 500 people may have been killed. The figure was based on estimates provided by local sources, the report said.
A doctor at a military hospital said at least 66 soldiers had died in the violence, but it wasn’t clear if they were fighting for or against the government.
"So far we have lost seven soldiers who died while they were waiting for medical attention and a further 59 who were killed outside," Ajak Bullen was quoted as saying.
Gunfire could be heard throughout the capital in the early hours of Tuesday.
Around 10,000 people, including women and children, have sought refuge at United Nations compounds since the fighting broke out on Sunday. Other residents have barricaded themselves in their homes or fled the city.
“We can still hear sporadic shooting from various locations. The situation is very tense,” Emma Jane Drew, who works for the British aid agency Oxfam in Juba, was quoted as saying.
“It’s continued shooting. Shooting could be heard all through the night. We don’t know who is fighting who.”
Kiir on Monday accused soldiers loyal to Machar of attempting a coup on Sunday. He said government troops were “in full control of the security situation in Juba” as he imposed an overnight curfew on the city. It proved ineffective as fighting continued.
South Sudan split from Sudan two years ago. The situation in the world’s newest country has been tense since Kiir sacked Machar along with his entire cabinet in July.
Earlier this month key members of the ruling party, the Sudan People's Liberation Movement, including Machar and Rebecca Garang, the widow of South Sudan's founding father John Garang, accused Kiir of being dictatorial.