Ugandan troops have also been sent to Somalia
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.