Fresh gunfire erupted overnight in the South Sudanese capital, Juba, a day after the government said it had quashed an attempted coup
There were reports of heavy weapons being fired near the presidential palace and in many other parts of Juba.
Several thousand people have taken refuge at two United Nations compounds.
On Monday, President Salva Kiir blamed soldiers loyal to ex-deputy Riek Machar - sacked in July - for the violence, in which at least 26 people died.
He said the clashes began when unidentified uniformed personnel opened fire at a meeting of the ruling party, former rebel force the Sudan People's Liberation Movement (SPLM), on Sunday night.
A night time curfew is now in place and at least four former ministers have reportedly been arrested after the alleged coup. Mr Kiir said the government was in full control of the capital.
Foreign Minister Barnaba Marial Benjamin said Mr Machar was "wanted by the government" and had gone into hiding.
President Kiir sacked Mr Machar, along with his whole cabinet, in July, reportedly following a power struggle.
Mr Machar, who has said he plans to contest the presidential elections in 2015, now leads a dissident faction within the SPLM.
On Tuesday, the foreign ministry said there was renewed fighting in Juba as the military "cleared out remnants" of Mr Machar's alleged supporters.
There were unconfirmed reports on local media of police raiding Mr Machar's residence. One report said tanks had surrounded his estate.
Hilde Johnson, the UN's special representative in the country, has urged all sides to exercise restraint and warned against community-motivated violence.
"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.
Ms Johnson's deputy, Toby Lanzer, said in a tweet that up to 13,000 people had sought shelter at two UN bases. Many of the civilians are woman and children.
Correspondents say UN staff have hunkered down in a bunker.
Juba's airport remains closed and telephone connections have been severely curtailed.
The government said some 120 people had been taken to a hospital in Juba for treatment.
Emma Jane Drew, the acting director of aid agency Oxfam's South Sudan branch, said she and her team were unable to leave their compound in Juba because of "continued shooting".
The UN and the US embassy advised their citizens to stay at home. Both denied rumours they were harbouring any political or military figures.
South Sudan - the world's youngest country and one of the least developed - has struggled to achieve a stable government since becoming independent from Sudan in 2011.
The independence referendum was intended to end a decade-long conflict, led by the SPLM, against the north.
But the oil-rich country remains ethnically and politically divided, with many armed groups active.