The UN mission to South Sudan has condemned Thursday's attack on its base in which two Indian peacekeepers and at least 11 civilians were killed.
Unmiss said the perpetrators of the "heinous crime" in Akobo, Jonglei state, must be brought to justice.
A similar attack is feared at another UN base where several thousand armed youths are reported to have gathered.
Clashes began in South Sudan when President Salva Kiir accused his ex-deputy of a failed coup a week ago.
The unrest has pitted gangs from the Nuer ethnic group of former Vice President Riek Machar against Dinkas - the majority group to which Mr Kiir belongs.
A number of countries have begun evacuating their nationals. Britain was sending another flight on Friday, a day after a military transport plane evacuated 182 people, including 53 Britons, to Uganda.
Uganda has sent a small contingent of troops to help pull out its nationals.
A US plane was also due to take Americans out of the country. And China's National Petroleum Company was evacuating oil workers to Juba.
France's ambassador to the UN, Gerard Araud, said there were fears of another assault as armed youths gathered near the UN compound in the town of Bor, in Jonglei, on Friday.
Earlier, Unmiss said two Indian peacekeepers were killed on and one injured on Thursday when some 2,000 armed youths believed to be of Nuer ethnicity surrounded the base in Akobo and opened fire "in the direction of Sudanese civilians of the Dinka ethnic group who had sought refuge in the compound".
Unmiss said at least 11 South Sudanese civilians also died. It had earlier put the death toll at 20.
Unmiss said all its personnel - along with civilians and members of non-governmental organisations - had now been airlifted from the base.
A memorial service will be held on Saturday in an Unmiss compound at the airport in the capital Juba.
Also on Friday, President Kiir told African mediators he agreed to "unconditional dialogue" to stop the violence.
He made the commitment during a meeting with foreign ministers from Ethiopia, Djibouti, Kenya, Sudan and Uganda.
President Kiir has blamed the violence on soldiers who support Mr Machar. Mr Machar, who was sacked by Mr Kiir in July, has denied trying to stage a coup. His whereabouts remain unknown.
Jonglei state has witnessed some of the worst violence since South Sudan became independent in 2011, with hundreds killed in periodic clashes between rival heavily-armed ethnic militias sparked by cattle-rustling.
Following decades of conflict, weapons - such as machine guns - are widely available in much of South Sudan.
Forces commanded by Gen Peter Gadet, who is loyal to Mr Machar, are said to be in control of the town of Bor - the capital of the Jonglei state.
Gen Gadet launched his campaign after reports of his fellow Nuers being killed.
South Sudan's government insists the clashes are over power and politics, not between ethnic groups.
The oil-rich country has struggled to achieve a stable government since becoming independent.