UN peacekeeping chief Herve Ladsous added that another 800 people have been wounded in battles between troops loyal to President Salva Kiir and an opposition leader, according to diplomats in the closed council consultations on the new crisis in Africa.
Rival troops are locked in fresh battles in South Sudan's capital in unrest that has killed at least 73 soldiers and sent thousands of terrified civilians fleeing since an alleged coup bid.
The fighting in the world's newest country has set off alarm bells in the international community, with the United Nations urging the warring groups to refrain from ethnic violence.
President Salva Kiir on Monday accused soldiers loyal to his arch-rival, former vice president Riek Machar, of staging a coup attempt in the oil-rich but deeply impoverished country.
The government said 10 key figures have been arrested, and that others, including Machar, were on the run.
Minister of Information Michael Makuei Lueth said that 73 soldiers were killed in the fighting, which erupted late on Sunday in the capital Juba.
Several civilians are also reported to have died.
Sporadic clashes could be heard throughout Tuesday in several parts of Juba.
As the city was locked down under a curfew late on Tuesday, Lueth said "full control" had been established, although sporadic gunfire could still be heard.
A top UN envoy said at least 10,000 civilians "have received protection in the two UNMISS compounds in Juba", and that UN staff were "taking every possible step to ensure their safety".
The special representative of the UN secretary-general, Hilde Johnson, said it was "paramount" that the conflict did not assume ethnic dimensions.
"At a time when unity among South Sudanese is more needed than ever, I call on the leaders of this new country and all political factions and parties, as well as community leaders to refrain from any action that fuels ethnic tensions and exacerbates violence," she said in a statement.
The African Union also said it was "deeply concerned" about the events and urged all players to show "maximum restraint".
The heavy fighting and alleged coup has underscored the fragility of the country which only became independent from Sudan in 2011.
Machar - who was sacked in July - leads a dissident group within South Sudan's ruling party, the Sudan People's Liberation Movement
(SPLM), and had been seen as the main challenger to Kiir.
The two men hail from different ethnic groups and had in the past fought on different sides during Sudan's long civil war.
An aid worker with the British charity Oxfam, Emma Jane Drew, said by telephone that the situation in Juba was "very tense".
"We don't know who is fighting who," she said, adding her team was unable to leave their compound.
The independent radio station Tamazuj said clashes were taking place around compounds belonging to Machar or his loyalists.
There were also unconfirmed reports of clashes between rival groups of soldiers elsewhere in the country.
The International Committee of the Red Cross (ICRC) said over the past two days more than 300 people have been admitted to Juba Teaching Hospital, which reported 26 deaths, and Juba Military Hospital.
Staff in both hospitals have been working around the clock, but they are struggling because of the sheer volume of patients and the severity of the injuries," said Felicity Gapes, an ICRC delegate leading the medical response on the ground.
Communications in Juba was sporadic, with most phone lines down.
