The announcement came as world leaders embarked on a diplomatic push to pull the world's youngest nation back from the brink of civil war.
Special envoys from the United States and Nigeria were expected in the capital Juba following a mission by foreign ministers from east Africa and the Horn.
The United States meanwhile announced that it had safely evacuated US nationals from the rebel-held city of Bor, a day after an aborted mission in which four US servicemen were injured when their aircraft came under fire in the flashpoint area.
The clashes have left hundreds dead and sent tens of thousands of people fleeing for protection in UN bases or to safer parts of the country, which only won independence from Sudan in 2011, but remains blighted by ethnic divisions, corruption and poverty.
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The fighting has both ethnic and political dimensions, as troops loyal to Kiir, an ethnic Dinka, battle forces backing Machar, a Nuer.
According to the UN, some 20 Dinka ethnic civilians were also killed in the assault.
The UN mission in South Sudan (UNMISS) said in a statement it would "reinforce its military presence in Bor and Pariang to continue fulfilling its mandate to help protect South Sudanese civilians".
Forces loyal to Machar are currently in control of Bor, the capital of Jonglei state and situated about 200 kilometres (125 miles) north of Juba, although South Sudan's army spokesman said government troops were advancing to retake the town.