He also said there was fighting about 70 kilometers (45 miles) north of the South Sudanese capital of Juba.
Heavy fighting erupted in Malakal, the capital of oil-producing Upper Nile state, which renegade forces briefly held before government troops retook it. The fighting began early Tuesday morning in the vicinity of the United Nations base in Malakal, with combatants using heavy machine guns and tanks, U.N. spokesman Martin Nesirky said at U.N. headquarters in New York.
Stray bullets are reported to have landed inside the U.N. base, wounding people who sought shelter there, according to Nesirky. As a result of Tuesday's violence, he said, the number of people seeking refuge at the U.N. base in Malakal has nearly doubled to 20,000.
South Sudan has a history of ethnic rivalry, and its many tribes have long battled each other in recurring cycles of violence. The fighting 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.
Nearly 10,000 people have been killed in the latest fighting, according to one estimate by an International Crisis Group analyst.
Some of the fiercest battles have been fought in Jonglei, South Sudan's largest state, where for months government troops had been trying to put down a local rebellion.
South Sudan's government says it has made peace with the leader of that rebellion, David Yau Yau, a renegade colonel from the Murle tribe who appears to have cut a deal with the Dinka-led government against Machar's mostly Nuer forces.
The Associated Press on Tuesday saw video showing a representative of Kiir's government meeting with Yau Yau earlier this week after the militia leader agreed to integrate his fighters into the national army. In the video bodies of men in combat fatigues litter the bushes, but it is impossible to tell if the dead are rebels or government troops.
The violence also appears to be drawing in neighboring Uganda, which supports Kiir's government. Ugandan officials say troops have been deployed to South Sudan to protect key installations such as Juba's airport and to facilitate civilian evacuations.
They deny Ugandan forces are involved in active combat, but a spokesman for the rebels, former South Sudan Brig. Gen. Lul Ruai Kong, said Ugandan helicopters and fighter jets are bombing rebel positions.
Another pro-rebel official, Gideon Gatpan Thaor, said fighters described being hit with a smoky weapon that burns, possibly white phosphorous.
Lt. Col. Paddy Ankunda, the Ugandan military spokesman, said Tuesday that Ugandan and South Sudanese army officials are drafting an agreement to extend their deployment. But he hinted that Ugandan troops would take on an official combat role, saying when the pact is signed "there could be things under the agreement which the forces might engage in."
Amid rumors some Ugandan forces have been killed or wounded in South Sudan, Ugandan lawmakers on Tuesday met for a special session to discuss the legality of the deployment.
Ugandan President Yoweri Museveni is highly influential in Juba, based in part on his decades-long support for the armed secessionist movement that eventually led to the creation in 2011 of the new state of South Sudan.
Muhumuza reported from Kampala, Uganda. Associated Press writer Edith M. Lederer at the U.N. contributed to this report.