Juba - South Sudanese rebels said on Monday they had shot down a Ugandan military helicopter fighting on the government side, killing the two pilots and the entire crew.
The statement by rebel spokesperson Dickson Gatluak did not give the number of those on board.
The rebels said the helicopter was downed early on Monday in Kodok near the northern oil town of Malakal, which has seen an upsurge in fighting in recent weeks.
South Sudanese army spokesperson Philip Aguer denied that a helicopter had been shot down, but admitted that Kodok was under rebel control.
Joseph Contreras, a spokesman for the United Nations mission in South Sudan, said he did not yet have information on the alleged incident.
Uganda has thousands of troops in South Sudan helping President Salva Kiir fight the rebels headed by his long-time political rival and former deputy, Riek Machar.
Rebels have previously claimed to have shot down Ugandan helicopters, but the claims have not been confirmed.
South Sudan is an important business partner for neighbouring Uganda, but thousands of Ugandans had to be evacuated from the country after the military conflict broke out in December 2013.
The conflict has killed tens of thousands and displaced about two million people.