Around 200 people, mostly women and children, have drowned after the boat they were using to flee fighting in South Sudan sank on the Nile.
Tens of thousands have already crossed the Nile to escape fighting around the rebel-held city of Bor.
A South Sudan government spokesman said the front line continues to shift but it appears fighting is about 45 miles north of Juba, the country’s capital.
In Ethiopia, where peace talks are taking place, a spokesman for the rebels said Ugandan helicopters and jets were bombing rebel positions.
As control of certain regions in South Sudan has changed, tens of thousands have fled their homes to escape fighting that often pits the Dinka ethnic group of president Salva Kiir against the Nuer group of Riek Machar, the former vice-president who now commands renegade forces.
The violence has displaced 413,000 people.
Troops from neighbouring Uganda appear to be fighting on behalf of President Kiir, who is reportedly seeking the long-term commitment of Ugandan troops in the fight against renegade forces. Uganda denies its forces are already involved in combat but admit that is where they are headed after the rebel threat to take Juba, where fighting began on December 15 before spreading across the country.
It now appears Ugandan president Yoweri Museveni has sent thousands of men and hardware that may have given government forces an edge against rebels. Pro-Machar rebels have since lost the oil-producing centres previously under their control.
Ugandan officials initially said troops were deployed to South Sudan to protect key installations such as the airport.