Between 400 and 500 people have been killed this week in clashes between Army units in South Sudan, U.N. officials quoted military sources as saying.

The government confirmed it had taken several politicians into custody, and was seeking to arrest former Vice-President Reik Machar, whom it accused of leading mutinous soldiers in an attempted coup.

Mr. Machar denied the allegations in an interview to online newspaper Sudan Tribune.

“I have no connection with or knowledge of any coup attempt,” Mr. Machar was quoted as saying, adding that South Sudan President Salva Kiir was becoming autocratic. “We don’t want him as the president of South Sudan any more.” Machar, who was dismissed by Kiir in July during a cabinet reshuffle that saw several other ministers lose their jobs, was expected to launch a campaign against the president in 2015 elections.

The clashes that started Sunday were starting to abate on Wednesday.

“This is certainly a calmer situation than at any time since Sunday. However, the crisis goes on, with fighting going on elsewhere in the country,” said Joseph Contreras, a spokesman for the U.N. mission in South Sudan (UNMISS).

The violence has spread to the eastern state of Jonglei, an area prone to ethnic clashes. There were additional reports of fighting in southern regions.

“The situation is very fluid and very volatile. I don’t think this crisis is coming to an end yet,” Mr. Contreras told DPA via telephone. “(The fact that) violence has spread to other states is worrying.”

The United Nations currently has no plans to evacuate its staff from the country.

The United States said it ordered the departure of all non-emergency staff from South Sudan, as well as offering American citizens seats on planes heading out of the country.

Britain’s Foreign Office said its embassy would remain open but that it was temporarily withdrawing some embassy staff and their dependants from Juba.

More than 16,500 civilians have sought shelter at the UNMISS compound in Juba. Mr. Contreras said the U.N. was starting to plan the delivery of humanitarian aid.

There appears to be a strong ethnic element to the fighting, pitting the Dinka and Nuer people against each other, but government officials have downplayed this.

South Sudan gained independence from Sudan in 2011 after years of civil war, but has witnessed widespread tribal violence in which hundreds have died.

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