Around 200 people, mostly women and children, have drowned after the boat they were using to flee fighting in Southern Sudan sank on the Nile.
Tens of thousands have already crossed the Nile in wooden boats to escape fighting around the rebel-held city of Bor.
A government military 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 fighter jets are bombing rebel positions.
As control of certain regions in South Sudan has changed, tens of thousands of residents 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, including more than 73,000 who sought refuge in neighboring countries, according to the United Nations.
Troops from neighbouring Uganda appear to be actively fighting on behalf of President Kiir, who is reportedly seeking the long-term commitment of Ugandan troops in the fight against renegade forces.
Uganda's military denies its forces are already involved in active combat but admit that is where they are headed following the rebels' threat to take Juba, where fighting erupted on December 15 before it spread 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 capitals previously under their control, giving Mr Kiir the upper hand against his rivals. Mr Kiir has refused to release the political detainees who are Mr Machar's allies, one of the conditions set by the rebels before they can sign any ceasefire deal with the government.
Ugandan officials initially said troops were deployed to South Sudan to protect key installations such as the airport as well as to facilitate civilian evacuations.
Amid rumors some Ugandan forces have been killed or wounded in South Sudan, Ugandan MPs met for a special session to discuss the legality of the deployment.