Uganda has acknowledged its forces are helping South Sudan's army battle rebels, reversing a previous denial.
A Ugandan military spokesman said Thursday his country's forces are helping to drive rebels out of Bor, the capital of South Sudan's Jonglei state.
Earlier, President Yoweri Museveni said Ugandan forces took part in a "big battle" with rebels Monday about 90 kilometers from the capital, Juba.
Uganda previously said its forces were present in South Sudan to protect and evacuate Ugandan civilians. Rebels had accused Uganda of fighting with soldiers loyal to President Salva Kiir.
In mid-December, President Kiir accused his former deputy, Riek Machar, of a coup attempt. Machar denied the allegations, but has called for the overthrow of Mr. Kiir.
South Sudan's spiraling violence has left at least 1,000 people dead and an estimated 500,000 displaced.
Meanwhile, Human Rights Watch says "appalling crimes" have been committed against civilians in South Sudan for "no other reason than their ethnicity."
Kiir is an ethnic Dinka while Machar is an ethnic Nuer. In a Thursday statement, Human Rights Watch said members of the army killed hundreds of Nuer men in Juba last month, while rebels targeted and killed civilians of Dinka origin in other parts of the country.