South Sudan says Sudanese warplanes dropped several bombs early Thursday in their first attack on a major southern town.
There has been no immediate confirmation of casualties.
South Sudan's Deputy Information Minister, Atem Yaak Atem, said five bombs dropped in the town of Bentiu were apparently aimed at a bridge.
Wednesday, Sudan announced it was pulling out of talks with South Sudan after South Sudan's army claimed control of the oil producing town of Heglig - a disputed border region.
Sudan has accused South Sudan of launching attacks in Heglig with the help of local rebels who have been fighting Khartoum since last June.
South Sudan claims its forces pursued Sudanese troops into Heglig after being attacked on Tuesday.
The African Union is trying to mediate bitter disputes between the two Sudans stemming from the south's independence last year. But the talks in Ethiopia's capital have made little progress.
Key issues include borders, the sharing of oil revenue and the status of nationals in each other's territory.
Both countries have suggested the possibility of renewed war. Before their separation, north and south Sudan fought a 21-year civil war that ended with the 2005 peace agreement.
Some information for this report was provided by AFP.