South Sudan's armed opposition is accusing government troops of attacking the northwestern town of Raga and killing 14 civilians.
Opposition spokesman Lam Paul Gabriel says troops attacked the town this morning.
Spokesmen for South Sudan's government and military could not immediately be reached for comment.
The fighting comes after residents of the western town of Wau reported ethnically targeted attacks on Monday, with the UN mission in South Sudan saying its workers had seen the bodies of 16 civilians.
South Sudan's three-year civil war has grown more intense in recent weeks. UN officials repeatedly have warned of the risk of genocide.
The acting envoy of the UN secretary-general, Moustapha Soumare, says both sides must show restraint and protect all South Sudanese citizens.