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.