UN Secretary General Ban Ki-moon on Monday called on the Security Council to implement an arms embargo immediately against South Sudan and to bolster a UN peace mission in the country.
Ban also called for sanctions against all parties that have hindered the implementation of a peace accord signed in August 2015. He said a decision over whether President Salva Kiir and Vice President Reik Machar should resign is up to the people of South Sudan.
Around 270 people were killed between Friday and Sunday in fighting that broke out just before the fifth anniversary of South Sudan's independence from Sudan, according to the government.
The Security Council signalled Sunday during a special session that it was prepared to strengthen its peace mission in South Sudan, which comprises more than 13,000 military personnel and police.
Kiir belongs to the Dinka, the largest and most influential ethnic group in southern Sudan. Machar is a Nuer, which is a slightly smaller group. The conflict in South Sudan has developed along these ethnic lines.
Since the end of 2013 tens of thousands of people have been killed and about 2.5 million other people have fled, according to the United Nations.
According to the UN refugee agency, thousands more have fled the new outbreak of violence and are stranded at Uganda's borders.
The agency is preparing for a mass influx of people, UNHCR spokesman Charles Yaxley told dpa. The agency has set up relief camps and made medical supplies available. Around two-thirds of the refugees are children, Yaxley said.