South Sudan's military said a boat carrying mostly women and children fleeing fighting in the northern part of the country has overturned, killing at least 200 people.
Spokesman Philip Aguer said Tuesday the accident happened on the White Nile river near the city of Malakal.
Also Tuesday, U.N. humanitarian official Toby Lanzer reported fresh fighting in and around Malakal, the capital of the oil-rich Upper Nile state.
Lanzer said the number of civilians seeking protection at the U.N. base in Malakal has risen to 19,000 from the 10,000 he reported there on Friday.
The warring sides in South Sudan were due to hold face-to-face talks again Tuesday in Ethiopia's capital, Addis Ababa, as they negotiate a possible cease-fire deal.
An agreement to halt a month of fighting has been held up by the government's refusal to release 11 political detainees, which rebels have demanded.
Army forces loyal to President Salva Kiir are battling soldiers who back former vice president Riek Machar, who Kiir fired in July.
The United Nations says more than 200,000 South Sudanese have been displaced from their homes since fighting began.
The conflict has sparked fears of a civil war in the world's newest country, which became independent from Sudan less than three years ago.