Witnesses and officials in South Sudan said rebels have attacked the capital of oil-rich Upper Nile state.
Toby Lanzer, the top U.N. humanitarian official in South Sudan, said the fighting broke out Tuesday morning in Malakal, and urged all those involved to respect people's rights and protect civilians.
Rebels once controlled Malakal, but South Sudan's military, which is being helped by forces from neighboring Uganda, retook the city.
South Sudan's government signed a peace agreement with rebels last month, but both sides have accused the other of breaking the pact.
The fighting began in mid-December, and has left thousands dead and forced more than 800,000 people from their homes.