Ugandan troops are fighting alongside South Sudanese government forces against rebels, President Yoweri Museveni has confirmed.
He said the combined forces had defeated rebels in a "big battle" north of the capital Juba.
Mr Museveni said some Ugandans had been killed but did not give any details.
Several thousand people are believed to have been killed over the past month in South Sudan in the conflict between the government and the rebels.
Army spokesman Lt Col Paddy Ankunda said Uganda has about two battalions, or 1,600 soldiers, in the country.
The announcement comes as fighting continues around the cities of Bor and Malakal - government forces are moving on Bor, while the rebels are trying to seize control of Malakal.
More than 350,000 people have been displaced by the fighting and 40,000 Ugandans have been evacuated
The conflict broke out on 15 December, when President Salva Kiir accused his former deputy Riek Machar of plotting a coup - charges he denies.
The dispute has seen killings along ethnic lines - Mr Kiir is a member of the Dinka community, the country's largest, while Mr Machar is from the Nuer ethnic group.
In a summit in Angola, Mr Museveni said: "Only the other day, 13 January, the SPLA [South Sudan army] and elements of our army had a big battle with these rebel troops at a point about 90 kilometres [55 miles] from Juba, where we inflicted a big defeat on them."
"Unfortunately, many lives were lost on the side of the rebels. We also took casualties and also had some dead."
Mr Museveni questioned why, if Mr Machar had not planned a coup, forces loyal to him had gone on to seize control of cities such as Bor.
Ugandan officials have previously said that their special forces were only in South Sudan to help evacuate their nationals.
Since South Sudan became independent in 2011, thousands of Ugandans have crossed the border to work or do business.
Some 40,000 Ugandan nationals have been evacuated since the conflict broke out.
On Tuesday, Uganda's parliament approved the decision to send troops to South Sudan.
Fighting erupted in the South Sudan capital, Juba, in mid-December. It followed a political power struggle between President Salva Kiir and his ex-deputy Riek Machar. The squabble has taken on an ethnic dimension as politicians' political bases are often ethnic.
Sudan's arid north is mainly home to Arabic-speaking Muslims. But in South Sudan there is no dominant culture. The Dinkas and the Nuers are the largest of more than 200 ethnic groups, each with its own languages and traditional beliefs, alongside Christianity and Islam.
Both Sudan and the South are reliant on oil revenue, which accounts for 98% of South Sudan's budget. They have fiercely disagreed over how to divide the oil wealth of the former united state - at one time production was shutdown for more than a year. Some 75% of the oil lies in the South but all the pipelines run north
The two Sudans are very different geographically. The great divide is visible even from space, as this Nasa satellite image shows. The northern states are a blanket of desert, broken only by the fertile Nile corridor. South Sudan is covered by green swathes of grassland, swamps and tropical forest.
After gaining independence in 2011, South Sudan is the world's newest country - and one of its poorest. Figures from 2010 show some 69% of households now have access to clean water - up from 48% in 2006. However, just 2% of households have water on the premises.
Just 29% of children attend primary school in South Sudan - however this is also an improvement on the 16% recorded in 2006. About 32% of primary-age boys attend, while just 25% of girls do. Overall, 64% of children who begin primary school reach the last grade.
Almost 28% of children under the age of five in South Sudan are moderately or severely underweight - this compares with the 33% recorded in 2006. Unity state has the highest proportion of children suffering malnourishment (46%), while Central Equatoria has the lowest (17%).
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