Britain's Foreign Office said about 150 of the estimated 500 Britons in South Sudan had contacted British officials, many of them saying they wanted to leave the Northeast-Central African country, the New York Times reported.
A day earlier the South Sudanese Army said it lost control of Bor in the north of the country. South Sudan's army acknowledged that it wasn't "in control of Bor" after forces loyal to Gen. Peter Gatdet Yak, who defected, stormed military bases Tuesday and seized the town Wednesday, the Sudan Tribune reported.
Observers and independent analysts expressed concerns that Gadet's defection could make security more difficult in Jonglei state, where government forces have been battling for control in areas where rebel David Yau been active since 2012.
During the past few days, President Salva Kiir accused his former vice president, Riek Machar, of attempting a coup, which Machar denied Wednesday. Tensions have been bubbling since July, when Kiir sacked his cabinet, including Machar.
Gerard Araud, the French ambassador to the United Nations and the current president of the Security Council, told the BBC the fighting in South Sudan "has the potential for civil war" between the country's two main ethnic groups, the Dinka and the Nuer.
Araud said the United Nations had 7,000 to 8,000 troops in South Sudan but their mission was to protect civilians, not engage in fighting.
In a release, the U.N. Mission in South Sudan said security conditions in Bor "have deteriorated significantly during the course of the day."
"UNMISS has received reports that heavy fighting erupted in the city in the wee hours of this morning and continued for four hours," the release said. "The violence triggered an exodus of civilians out of Bor, and thousands have sought shelter at the mission's compound on the southeastern outskirts of the city."
U.S. President Barack Obama said in a statement late Thursday the fighting "threatens to plunge South Sudan back into the dark days of its past."
"Now is the time for South Sudan's leaders to show courage and leadership, to reaffirm their commitment to peace, to unity, and to a better future for their people. The United States will remain a steady partner of the South Sudanese people as they seek the security and prosperity they deserve," the White House statement said.