Warfare in the world's newest state has displaced more than 400,000 people since mid-December.
The number of people seeking refuge at a UN camp in Malakal has nearly doubled to 20,000.
Stray bullets are reported to have landed inside the UN camp, wounding people who sought shelter there.
The UN mission in South Sudan said it was "gravely concerned about mounting evidence of gross violations of international human rights law" across the country.
Nearly 10,000 people have been killed in the latest fighting, according to one estimate.

sudan baby hospital
A displaced mother carries her sick child at a United Nations hospital at Tomping campReuters
refugee camp drainage ditch
Displaced people wash their clothes in a drainage canal at Tomping campReuters
sudan baby bath
An internally displaced girl bathes a younger child at Tomping campReuters
refugee camp water
Displaced people wait for water at Tomping refugee campReuters
women water bottles
Displaced people carry water containers on their heads at Tomping campReuters
UN water pipe
United Nations Mission in South Sudan (UNMISS) peacekeepers from Japan assemble a drainage pipe at Tomping campReuters
sudan soldier
A South Sudanese soldier, adorned with a bandolier of bullets, listens during a briefing at the army general headquarters in JubaReuters
sudan soldiers sing
SPLA soldiers sing as they celebrate regaining the rebel-held town of Bentiu, restoring government control of Unity state where oil production had been halted by fightingReuters
sudan refugees boat
Refugees from Bor county unload their belongings from a boat after arriving at Minkaman port in Awerial countyReuters
refugees belongings
People displaced by the fighting in Bor county carry their belongings after arriving at the port of Minkaman, in Awerial countyReuters
refugees women
Two woman displaced by the fighting in Bor county chat as they sit by their mosquito nets in Minkaman, Awerial countyReuters
sudan women hug
Women displaced by the fighting in Bor county hug each other at the port in Minkaman, Awerial countyReuters