Fighting took place for the first time since the two sides signed a cease-fire agreement in January, the BBC reported.
"Hostilities have this morning broken out in Malakal: all parties engaged in the violence must uphold people's rights and protect non-combatants," U.N. Humanitarian Coordinator in South Sudan, Toby Lanzer, said in a Twitter post
The fighting fuels concerns over the safety of oil fields in the north, the backbone of the country's economy, the BBC said.
Clashes between government forces and rebel groups initially broke out Dec. 15 after Machar allegedly attempted to stage a coup. Machar, who was pushed out of office when President Salva Kiir reshuffled his Cabinet in July, has denied the coup allegations.
A top South Sudanese official said Monday that 70 percent of the country's troops have defected to the rebel forces and are now loyal to Machar, the Sudan Tribune reported.