Fighting first broke out in December 2013, when President Salva Kiir accused his former deputy Riek Machar of attempting a coup, which he denies.
The UN's aid chief in South Sudan, Toby Lanzer, said the conflict had now resumed in the oil-rich Unity state.
Areas around the regional capital, Bentiu, were worst-hit, he said.
Mr Lanzer added that the fighting was taking place at the peak of planting season, when people need to move around the region to help make a living.
Mr Kiir and Mr Machar signed a ceasefire deal in January this year, but violence has continued in parts of the country.
The UN estimated 10,000 people have been killed and another 1.5 million displaced since the conflict began.
Mr Kiir's term as president ends in July, four years after South Sudan gained independence to become the world's newest country.
Close to 100,000 people have been displaced by renewed fighting in South Sudan in the last week, the United Nations has warned.