The United Nations began evacuating "non-critical staff" from Juba, the capital of South Sudan, and will send military reinforcements to embattled towns further north, it said Sunday.

The UN mission in South Sudan (UNMISS) said the relocation of its staff to Uganda was a "precautionary measure to reduce pressures on its limited resources" in Juba, where nearly 20,000 civilians have taken refuge at the UN.

The statement did not specify how many personnel will be affected by the evacuation order.

Civilian staff at the UN base in Bor, 200 kilometers (125 miles) north of Juba, have been transferred to the capital, and UNMISS will "reinforce its military presence in Bor and Pariang to continue fulfilling its mandate to help protect South Sudanese civilians," the statement said.

At the same time, preparations are under way to evacuate non-critical personnel from the UN base in Bentiu, around 1,000 kilometers north of Juba, and to send in military reinforcements.

"We are not abandoning South Sudan," said the Special Representative of the UN Secretary-General for South Sudan, Hilde F. Johnson.

"To anyone who wants to threaten us, attack us or put obstacles in our way, our message remains loud and clear: we will not be intimidated."

A UN base was attacked Thursday in Akobo, in Jonglei state, by young ethnic Nuers who killed a dozen civilians of the rival Dinka ethnic group, as well as two Indian peacekeepers.

Since December 15, an armed conflict has escalated between the forces of the former vice-president Riek Machar and those of President Salva Kiir.

In Juba, the clashes have left at least 500 people dead and, across the country, tens of thousands of people have been displaced by the fighting, according to incomplete figures.

Machar and Kiir, both veterans of the long war for independence that stretched from 1983 to 2005, are using militants from their respective ethnic groups -- Dinka for Kiir and Nuer for Machar.